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OUTLINES  Ol^ 

j"he  earth's. 
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OUTLINES    OF    THE 
EARTH'S     HISTORY 


A  POPULAR  STUDY 
IN    PHYSIOGRAPHY 


BY 
NATHANIEL  SOUTHGATE   SHALER 

PROFESSOR   OF   GEOLOGY    IN    HARVARD   UNIVERSITY 
DEAN    OF   LAWRENCE   SCIENTIFIC    SCHOOL 


ILLUSTRATED 
WITH   INDEX 


4 


NEW   YORK   AND   LONDON 
D.  APPLETON   AND   COiMPANY 

1910 


<^ 


^oy'^ 


Copyright,  1898, 
By  D.  APPLETON  AND  COMPANY. 


PREFACE 


The  object  of  this  book  is  to  provide  the  beginner  in 
the  study  of  the  earth's  history  with  a  general  account  of 
those  actions  which  can  be  readily  understood  and  which 
will  afford  him  clear  understandings  as  to  the  nature  of 
the  processes  which  have  made  this  and  other  celestial 
spheres.  It  has  been  the  writer's  purpose  to  select  those 
series  of  facts  which  serve  to  show  the  continuous  opera- 
tions of  energy,  so  that  the  reader  might  be  helped  to  a 
truer  conception  of  the  nature  of  this  sphere  than  he  can 
obtain  from  ordinary  text-books. 

In  the  usual  method  of  presenting  the  elements  of  the 
earth's  history  the  facts  are  set  forth  in  a  manner  which 
leads  the  student  to  conceive  that  history  as  in  a  way 
completed.  The  natural  prepossession  to  the  effect  that 
the  visible  universe  represents  something  done,  rather 
than  something  endlessly  doing,  is  thus  re-enforced,  with 
the  result  that  one  may  fail  to  gain  the  largest  and  most 
educative  impression  which  physical  science  can  afford  him 
in  the  sense  of  the  swift  and  unending  procession  of 
events. 

It  is  well  known  to  all  who  are  acquainted  with  the  his- 
tory of  geology  that  the  static  conception  of  the  earth — 
the  idea  that  its  existing  condition  is  the  finished  product 
of  forces  no  longer  in  action — led  to  prejudices  which 
have  long  retarded,  and  indeed  still  retard,  the  progress 
of  that  science.  This  fact  indicates  that  at  the  outset 
of  a  student's  work  in  this  field  he  should  be  guarded 

iii 

335962 


iv  OUTLINES  OF  THE  EARTH'S  HISTORY. 

against  such  misconceptions.  The  only  way  to  attain  the 
end  is  by  bringing  to  the  understanding  of  the  beginner 
a  clear  idea  of  successions  of  events  which  are  caused  by 
the  forces  operating  in  and  on  this  sphere.  Of  all  the 
chapters  of  this  great  story,  that  which  relates  to  the  his- 
tory of  the  work  done  by  the  heat  of  the  sun  is  the  most 
interesting  and  awakening.  Therefore  an  effort  has  been 
made  to  present  the  great  successive  steps  by  which  the 
solar  energy  acts  in  the  processes  of  the  air  and  the  waters. 
~  The  interest  of  the  beginner  in  geology  is  sure  to  be 
aroused  when  he  comes  to  see  how  very  far  the  history  of 
the  earth  has  influenced  the  fate  of  men.  Therefore  the 
aim  has  been,  where  possible,  to  show  the  ways  in  which 
geological  processes  and  results  are  related  to  ourselves; 
how,  in  a  word,  this  earth  has  been  the  well-appointed 
nursery  of  our  kind. 

All  those  who  are  engaged  in  teaching  elementary 
science  learn  the  need  of  limiting  the  story  they  have  to 
tell  to  those  truths  which  can  be  easily  understood  by 
beginners.  It  is  sometimes  best,  as  in  stating  such  diffi- 
cult matters  as  those  concerning  the  tides,  to  give  explana- 
tions which  are  far  from  complete,  and  which,  as  to  their 
mode  of  presentation,  would  be  open  to  criticism  were  it 
not  for  the  fact  that  any  more  elaborate  statements  would 
most  likely  be  incomprehensible  to  the  novice,  thus  de- 
feating the  teacher's  aim. 

It  will  be  observed  that  no  account  is  here  given  of 

the  geological  ages  or  of  the  successions  of  organic  life. 

Chapters    on    these    subjects    were    prepared,    but    were 

omitted  for  the  reason  that  they  made  the  story  too  long, 

and  also  because  they  carried  the  reader  into  a  field  of 

much  greater  difficulty  than  that  which  is  found  in  the 

physical  history  of  the  earth. 

N.  S.  S. 
March,  1898. 


CONTENTS 


CHAPTER 

I. — An  introductiox  to  the  study  of  Nature 
II. — Ways  and  means  of  studying  Nature 
III. — The  stellar  realm  .... 

IV. — The  earth 

V. — The  ATMosrHERE        .... 

VI. — Glaciers 

VII. — The  work  of  underground  water 

VI II.— The  soil 

IX. — The  rocks  and  their  order  . 


PAGE 

1 
Q 

31 
81 
97 
207 
250 
313 
349 


LIST   OF  FULL-PAGE   ILLUSTRATIONS. 


PAQB 

Dunes  at  Ipswich  Light,  Massachusetts       .        .        Frontispiece 

Seal  Rocks  near  San  Francisco,  California 33 

Lava  stream,  in  Hawaiian  Islands,  flowing  into  the  sea  .        ,  72 

Waterfall  near  Gadsden,  Alabama 90 

South  shore,  Martha's  Vineyard,  Massachusetts  .        .        .        .121 

Pocket  Creek,  Cape  Ann,  Massachusetts 163 

Muir  Glacier,  Alaska      .        .     ' 207 

Front  of  Muir  Glacier 240 

Mount  ^tna,  seen  from  near  Catania 291 

Mountain  gorge,  Himalayas,  India       .•••..    330 

vii 


OUTLINES  OF  THE  EARTH^S  HISTORY. 


CHAPTER  I. 

AN   INTRODUCTION   TO   THE   STUDY   OF   NATURE. 

The  object  of  this  book  is  to  give  the  student  who  is 
about  to  enter  on  the  study  of  natural  science  some  gen- 
eral idea  as  to  the  conditions  of  the  natural  realm.  As 
this  field  of  inquiry  is  vast,  it  will  be  possible  only  to  give 
the  merest  outline  of  its  subject-matter,  noting  those  fea- 
tures alone  which  are  of  surpassing  interest,  which  are 
demanded  for  a  large  understanding  of  man's  place  in  this 
world,  or  which  pertain  to  his  duties  in  life. 

In  entering  on  any  field  of  inquiry,  it  is  most  desirable 
that  the  student  should  obtain  some  idea  as  to  the  ways 
in  which  men  have  been  led  to  the  knowledge  which  they 
possess  concerning  the  w^orld  about  them.  Therefore  it 
will  be  well  briefly  to  sketch  the  steps  by  which  natural 
science  has  come  to  be  what  it  is.  By  so  doing  we  shall 
perceive  how  much  we  owe  to  the  students  of  other  gen- 
erations; and  by  noting  the  difficulties  which  they  encoun- 
tered, and  how  they  avoided  them,  we  shall  more  easily  find 
our  own  way  to  knowledge. 

The  primitive  savages,  who  were  the  ancestors  of  all 
men,  however  civilized  they  may  be,  were  students  of  Na- 
ture. The  remnants  of  these  lowly  people  who  were  left 
in  different  parts  of  the  world  show  us  that  man  was  not 
long  in  existence  before  he  began  to  devise  some  explana- 
tion concerning  the  course  of  events  in  the  outer  world. 

1 


2  OUTLINJeS  OF  THE  EARTH'S  HISTORY. 

Seeing  the  sun  rise  and  set,  the  changes  of  the  moon,  the 
alternation  of  the  seasons,  the  incessant  movement  of  the 
streams  and  sea,  and  the  other  more  or  less  orderly  suc- 
cessions of  events,  our  primitive  forefathers  were  driven 
to  invent  some  explanation  of  them.  This,  independently, 
and  in  many  different  times  and  places,  they  did  in  a  sim- 
ple and  natural  way  by  supposing  that  the  world  was  con- 
trolled by  a  host  of  intelligent  beings,  each  of  which  had 
some  part  in  ordering  material  things.  Sometimes  these 
invisible  powers  were  believed  to  be  the  spirits  of  great 
chieftains,  who  were  active  when  on  earth,  and  who  after 
death  continued  to  exercise  their  power  in  the  larger 
realms  of  Nature.  Again,  and  perhaps  more  commonly, 
these  movements  of  Nature  were  supposed  to  be  due  to 
the  action  of  great  though  invisible  beasts,  much  like  those 
which  the  savage  found  about  him.  Thus  among  our 
North  American  Indians  the  winds  are  explained  by  the 
supposition  that  the  air  is  fanned  by  the  wings  of  a  great 
unseen  bird,  whose  duty  it  is  to  set  the  atmosphere  into 
motion.  That  no  one  has  ever  seen  the  bird  doing  the 
work,  or  that  the  task  is  too  great  for  any  conceivable  bird, 
is  to  the  simple,  uncultivated  man  no  objection  to  this 
view.  It  is  long,  indeed,  before  education  brings  men  to 
the  point  where  they  can  criticise  their  first  explanations 
of  Nature. 

As  men  in  their  advance  come  to  see  how  much  nobler 
are  their  own  natures  than  those  of  the  lower  animals,  they 
gradually  put  aside  the  explanation  of  events  by  the  ac- 
tions of  beasts,  and  account  for  the  order  of  the  world  by 
the  supposition  that  each  and  every  important  detail  is 
controlled  by  some  immortal  creature  essentially  like  a  man, 
though  much  more  powerful  than  those  of  their  own  kind. 
This  stage  of  understanding  is  perhaps  best  shown  by  the 
mythology  of  the  Greeks,  where  there  was  a  great  god  over 
all,  very  powerful  but  not  omnipotent;  and  beneath  him, 
in  endless  successions  of  command,  subordinate  powers, 
each  with  a  less  range  of  duties  and  capacities  than  those  of 


AN  INTRODUCTION  TO  THE  STUDY  OF  NATURE.     3 

higher  estate,  until  at  the  bottom  of  the  system  there  were 
minor  deities  and  demigods  charged  with  the  management 
of  the  trees,  the  flowers,  and  the  springs — creatures  differ- 
ing little  from  man,  except  that  they  were  immortal,  and 
generally  invisible,  though  they,  like  all  the  other  deities, 
might  at  their  will  display  themselves  to  the  human  beings 
over  whom  they  watched,  and  whose  path  in  life  they 
guided. 

Among  only  one  people  do  we  find  that  the  process  of 
advance  led  beyond  this  early  and  simple  method  of  ac- 
counting for  the  processes  of  Nature,  bringing  men  to  an 
understanding  such  as  we  now  possess.  This  great  task  was 
accomplished  by  the  Greeks  alone.  About  twenty-five  hun- 
dred years  ago  the  philosophers  of  Greece  began  to  perceive 
that  the  early  notion  as  to  the  guidance  of  the  world  by 
creatures  essentially  like  men  could  not  be  accepted,  and 
must  be  replaced  by  some  other  view  which  would  more 
effectively  account  for  the  facts.  This  end  they  attained 
by  steps  which  can  not  well  be  related  here,  but  which  led 
them  to  suppose  separate  powers  behind  each  of  the  natural 
series — powers  having  no  relation  to  the  qualities  of  man- 
kind, but  ever  acting  to  a  definite  end.  Thus  Plato,  who 
represents  most  clearly  this  advance  in  the  interpretation 
of  facts,  imagined  that  each  particular  kind  of  plant  or  ani- 
mal had  its  shape  inevitably  determined  by  something 
which  he  termed  an  idea,  a  shape-giving  power  which 
existed  before  the  object  was  created,  and  which  would 
remain  after  it  had  been  destroyed,  ever  ready  again  to 
bring  matter  to  the  particular  form.  From  this  stage  of 
understanding  it  was  but  a  short  step  to  the  modern  view 
of  natural  law.  This  last  important  advance  was  made  by 
the  great  philosopher  Aristotle,  who,  though  he  died  about 
twenty-two  hundred  years  ago,  deserves  to  be  accounted 
the  first  and  in  many  ways  the  greatest  of  the  ancient  men 
of  science  who  were  informed  with  the  modern  spirit. 

With  Aristotle,  as  with  all  his  intellectual  successors, 
the  operations  of  Nature  were  conceived  as  to  be  accounted 


4  OUTLINES  OF  THE  EARTH'S  HISTORY. 

for  by  the  action  of  forces  which  we  commonly  designate 
as  natural  laws,  of  which  perhaps  the  most  familiar  and 
universal  is  that  of  gravitation,  which  impels  all  bodies 
to  move  toward  each  other  with  a  degree  of  intensity 
w^hich  is  measured  by  their  weight  and  the  distance  by 
which  they  are  separated. 

For  many  centuries  students  used  the  term  law  in 
somewhat  the  same  way  as  the  more  philosophical  believers 
in  polytheism  spoke  of  their  gods,  or  as  Plato  of  the  ideas 
which  he  conceived  to  control  Nature.  We  see  by  this 
instance  how  hard  it  is  to  get  rid  of  old  ways  of  thinking. 
Even  when  the  new  have  been  adopted  we  very  often  find 
that  something  of  the  ancient  and  discarded  notions  cling 
in  our  phrases.  The  more  advanced  of  our  modern  philos- 
ophers are  clear  in  their  mind  that  all  we  know  as  to  the 
order  of  Nature  is  that,  given  certain  conditions,  certain 
consequences  inevitably  follow. 

Although  the  limitations  which  modern  men  of  science 
perceive  to  be  put  upon  their  labours  may  seem  at  first 
sight  calculated  to  confine  our  understanding  within  a 
narrow  field  of  things  which  can  be  seen,  or  in  some  way 
distinctly  proved  to  exist,  the  effect  of  this  limitation  has 
been  to  make  science  what  it  is — a  realm  of  things  known 
as  distinct  from  things  which  may  be  imagined.  All  the 
difference  between  ancient  science  and  modern  consists  in 
the  fact  that  in  modern  science  inquirers  demand  a  busi- 
nesslike method  in  the  interpretation  of  Nature.  Among 
the  Greeks  the  philosopher  who  taught  explanations  of 
any  feature  in  the  material  world  which  interested  him 
was  content  if  he  could  imagine  some  way  which  would 
account  for  the  facts.  It  is  the  modern  custom  now  to 
term  the  supposition  of  an  explanation  a  working  hypothe- 
sis, and  only  to  give  it  the  name  of  theory  after  a  very 
careful  search  has  shown  that  all  the  facts  which  can  be 
gathered  are  in  accordance  with  the  view.  Thus  when 
Newton  made  his  great  suggestion  concerning  the  law  of 
gravitation,  which  was  to  the  effect  that  all  bodies  attracted 


AN  INTRODUCTION  TO  THE  STUDY   OF  NATURE.      5 

each  other  in  proportion  to  their  masses,  and  inversely  as 
the  square  of  their  distance  from  each  other,  he  did  not 
rest  content,  as  the  old  Greeks  would  have  done,  with  the 
probable  truth  of  the  explanation,  but  carefully  explored 
the  movements  of  the  planets  and  satellites  of  the  solar 
system  to  see  if  the  facts  accorded  with  the  hypothesis. 
Even  the  perfect  correspondence  which  he  found  did  not 
entirely  content  inquirers,  and  in  this  century  very  impor- 
tant experiments  have  been  made  which  have  served  to 
show  that  a  ball  suspended  in  front  of  a  precipice  will  be 
attracted  toward  the  steep,  and  that  even  a  mass  of  lead 
some  tons  in  weight  will  attract  toward  itself  a  small 
body  suspended  in  the  manner  of  a  pendulum. 

It  is  this  incessant  revision  of  the  facts,  in  order  to 
see  if  they  accord  with  the  assumed  rule  or  law,  which 
has  given  modern  science  the  sound  footing  that  it  lacked 
in  earlier  days,  and  which  has  permitted  our  learning  to 
go  on  step  by  step  in  a  safe  way  up  the  heights  to  which 
it  has  climbed.  All  explanations  of  Nature  begin  with 
the  work  of  the  imagination.  In  common  phrase,  they 
all  are  guesses  which  have  at  first  but  little  value,  and 
only  attain  importance  in  proportion  as  they  are  verified 
by  long-continued  criticism,  which  has  for  its  object  to 
see  whether  the  facts  accord  with  the  theory.  It  is  in 
this  effort  to  secure  proof  that  modern  science  has  gathered 
the  enormous  store  of  well-ascertained  facts  which  con- 
stitutes its  true  wealth,  and  which  distinguishes  it  from 
the  earlier  imaginative  and  to  a  great  extent  unproved 
views. 

In  the  original  state  of  learning,  natural  science  was 
confounded  with  political  and  social  tradition,  with  the 
precepts  of  duty  which  constitute  the  law  of  the  people, 
as  well  as  with  their  religion,  the  whole  being  in  the 
possession  of  the  priests  or  wise  men.  So  long  as  natural 
action  was  supposed  to  be  in  the  immediate  control  of 
numerous  gods  and  demigods,  so  long,  in  a  word,  as  the 
explanation  of  Nature  was  what  we  term  polytheistic,  this 


6  OUTLINES  OF  THE  EARTH'S  HISTORY. 

association  of  science  with  other  forms  of  learning  was 
not  only  natural  but  inevitable.  Gradually,  however,  as 
the  conception  of  natural  law  replaced  the  earlier  idea 
as  to  the  intervention  of  a  spirit,  science  departed  from 
other  forms  of  lore  and  came  to  possess  a  field  to  itself. 
At  first  it  was  one  body  of  learning.  The  naturalists  of 
Aristotle's  time,  and  from  his  day  down  to  near  our  own, 
generally  concerned  themselves  with  the  whole  field  of 
Nature.  For  a  time  it  was  possible  for  any  one  able  and 
laborious  man  to  know  all  which  had  been  ascertained  con- 
cerning astronomy,  chemistry,  geology,  as  well  as  the 
facts  relating  to  living  beings.  The  more,  however,  as 
observation  accumulated,  and  the  store  of  facts  increased, 
it  became  difficult  for  any  one  man  to  know  the  whole. 
Hence  it  has  come  about  that  in  our  own  time  natural 
learning  is  divided  into  many  distinct  provinces,  each  of 
which  demands  a  lifetime  of  labour  from  those  who  would 
know  what  has  already  been  done  in  the  field,  and  what 
it  is  now  important  to  do  in  the  way  of  new  inquiries. 

The  large  divisions  which  naturalists  have  usually  made 
of  their  tasks  rest  in  the  main  on  the  natural  partitions 
which  we  may  readily  observe  in  the  phenomenal  world. 
First  of  all  conies  astronomy,  including  the  phenomena 
exhibited  in  the  heavens,  beyond  the  limits  of  the  earth's 
atmosphere.  Second,  geology,  which  takes  account  of  all 
those  actions  which  in  process  of  time  have  been  devel- 
oped in  our  own  sphere.  Third,  physics,  which  is  con- 
cerned with  the  laws  of  energy,  or  those  conditions  which 
affect  the  motion  of  bodies,  and  the  changes  which  are 
impressed  upon  them  by  the  different  natural  forces. 
Fourth,  chemistry,  which  seeks  to  interpret  the  principles' 
which  determine  the  combination  of  atoms  and  the  mole- 
cules which  are  built  of  them  under  the  influence  of  the 
chemical  affinities.  Fifth,  biology,  or  the  laws  of  life,  a 
study  which  pertains  to  the  forms  and  structures  of  animals 
and  plants,  and  their  wonderful  successions  in  the  history 
of  the  world.     Sixth,  mathematics,  or  the  science  of  spacQ 


AN  INTRODUCTION  TO  THE  STUDY  OF  NATURE.      7 

and  number,  that  deals  with  the  principles  which  under- 
lie the  order  of  Nature  as  expressed  at  once  in  the  human 
understanding  and  in  the  material  universe.  By  its  use 
men  were  made  able  to  calculate,  as  in  arithmetic,  the 
problems  which  concern  their  ordinary  business,  as  well 
as  to  compute  the  movements  of  the  celestial  bodies,  and 
a  host  of  actions  which  take  place  on  the  earth  that  would 
be  inexplicable  except  by  the  aid  of  this  science.  Last  of 
all  among  the  primary  sciences  we  may  name  that  of  psy- 
chology, which  takes  account  of  mental  operations  among 
man  and  his  lower  kindred,  the  animals. 

In  addition  to  the  seven  sciences  above  mentioned, 
which  rest  in  a  great  measure  on  the  natural  divisions  of 
phenomena,  there  are  many,  indeed,  indefinitely  numerous, 
subdivisions  which  have  been  made  to  suit  the  convenience 
of  students.  Thus  astronomy  is  often  separated  into 
physical  and  mathematical  divisions,  which  take  account 
either  of  the  physical  phenomena  exhibited  by  the  heavenly 
bodies  or  of  their  motions.  In  geology  there  are  half  a 
dozen  divisions  relating  to  particular  branches  of  that 
subject.  In  the  realm  of  organic  life,  in  chemistry,  and 
in  physics  there  are  many  parts  of  these  sciences  which 
have  received  particular  names. 

It  must  not  be  supposed  that  these  sciences  have  the 
independence  of  each  other  which  their  separate  names 
would  imply.  In  fact,  the  student  of  each,  however,  far 
he  may  succeed  in  separating  his  field  from  that  of  the 
other  naturalists,  as  we  may  fitly  term  all  students  of 
Nature,  is  compelled  from  time  to  time  to  call  in  the  aid 
of  his  brethren  who  cultivate  other  branches  of  learning. 
The  modern  astronomer  needs  to  know  much  of  chemistry, 
or  else  he  can  not  understand  many  of  his  observations 
on  the  sun.  The  geologists  have  to  share  their  work  with 
the  student  of  animal  and  vegetable  life,  with  the  physi- 
cists; they  must,  moreover,  know  something  of  the  celes- 
tial spheres  in  order  to  interpret  the  history  of  the  earth. 
In  fact,  day  by  day,  with  the  advance  of  learning,  we  come 
2 


8  OUTLINES  OP  THE  EARTH'S  HISTORY. 

more  clearly  to  perceive  that  all  the  processes  of  Nature 
are  in  a  way  related  to  each  other,  and  that  in  proportion 
as  we  understand  any  part  of  the  great  mechanism,  we 
are  forced  in  a  manner  to  comprehend  the  whole.  In 
other  words,  we  are  coming  to  understand  that  these  divi- 
sions of  the  field  of  science  depend  upon  the  limitations 
of  our  knowledge,  and  not  upon  the  order  of  Nature  itself. 
For  the  purposes  of  education  it  is  important  that 
every  one  should  know  something  of  the  great  truths  which 
each  science  has  disclosed.  No  mortal  man  can  compass 
the  whole  realm  of  this  knowledge,  but  every  one  can  gain 
some  idea  of  the  larger  truths  which  may  help  him  to 
understand  the  beauty  and  grandeur  of  the  sphere  in  which 
he  dwells,  which  will  enable  him  the  better  to  meet  the 
ordinary  duties  of  life,  that  in  almost  all  cases  are  related 
to  the  facts  of  the  world  about  us.  It  has  been  of  late 
the  custom  to  term  this  body  of  general  knowledge  which 
takes  account  of  the  more  evident  facts  and  important 
series  of  terrestrial  actions  physiography,  or,  as  the  term 
implies,  a  description  of  Nature,  with  the  understanding 
that  the  knowledge  chosen  for  the  account  is  that  which 
most  intimately  concerns  the  student  who  seeks  informa- 
tion that  is  at  once  general  and  important.  Therefore, 
in  this  book  the  effort  is  made  first  to  give  an  account  as 
to  the  ways  and  means  which  have  led  to  our  understand- 
ing of  scientific  problems,  the  methods  by  which  each  per- 
son may  make  himself  an  inquirer,  and  the  outline  of  the 
knowledge  that  has  been  gathered  since  men  first  began 
to  observe  and  criticise  the  revelations  the  universe  may 
afford  them. 


CHAPTER  II. 

WAYS   AND   MEANS   OF   STUDYING   NATURE. 

It  is  desirable  that  the  student  of  Nature  keep  well 
in  mind  the  means  whereby  he  is  able  to  perceive  what 
goes  on  in  the  world  about  him.  He  should  understand 
something  as  to  the  nature  of  his  senses,  and  the  extent 
to  which  these  capacities  enable  him  to  discern  the  opera- 
tions of  Nature.  Man,  in  common  with  his  lower  kindred, 
is,  by  the  mechanism  of  the  body,  provided  with  five  some- 
what different  ways  by  which  he  may  learn  something 
of  the  things  about  him.  The  simplest  of  these  capacities 
is  that  of  touch,  a  faculty  that  is  common  to  the  general 
surface  of  the  body,  and  which  informs  us  when  the  sur- 
face is  affected  by  contact  with  some  external  object.  It 
also  enables  us  to  discern  differences  of  temperature.  Next 
is  the  sense  of  taste,  which  is  limited  to  the  mouth  and  the 
parts  about  it.  This  sense  is  in  a  way  related  to  that  of 
touch,  for  the  reason  that  it  depends  on  the  contact  of  our 
body  with  material  things.  Third  is  the  sense  of  smell, 
so  closely  related  to  that  of  taste  that  it  is  difficult  to  draw 
the  line  between  the  two.  Yet  through  the  apparatus  of 
the  nose  we  can  perceive  the  microscopically  small  parts 
of  matter  borne  to  us  through  the  air,  which  could  not  be 
appreciated  by  the  nerves  of  the  mouth.  Fourth  in  order 
of  scope  comes  the  hearing,  which  gives  us  an  account  of 
those  waves  of  matter  that  we  understand  as  sound.  This 
power  is  much  more  far  ranging  than  those  before  noted; 
in  some  cases,  as  in  that  of  the  volcanic  explosions  from 
the  island  of  Krakatoa,  in  the  eruption  of  1883,  the  con- 

9 


10  OUTLINES  OF  THE  EARTH'S  HISTORY. 

vulsions  were  audible  at  the  distance  of  more  than  a 
thousand  miles  away.  The  greater  cannon  of  modern 
days  may  be  heard  at  the  distance  of  more  than  a  hundred 
miles,  so  that  while  the  sense  of  touch,  taste,  and  smell 
demand  contact  with  the  bodies  which  we  appreciate,  hear- 
ing gives  us  information  concerning  objects  at  a  consider- 
able distance.  Last  and  highest  of  the  senses,  vastly  the 
most  important  in  all  that  relates  to  our  understanding 
of  Nature,  is  sight,  or  the  capacity  which  enables  us  to  ap- 
preciate the  movement  of  those  very  small  waves  of  ether 
which  constitute  light.  The  eminent  peculiarity  of  sight 
is  that  it  may  give  us  information  concerning  things  which 
are  inconceivably  far  away;  it  enables  us  to  discern  the 
light  of  suns  probably  millions  of  times  as  remote  from 
us  as  is  the  centre  of  our  own  solar  system. 

Although  much  of  the  pleasure  which  the  world 
affords  us  comes  through  the  other  senses,  the  basis  of 
almost  all  our  accurate  knowledge  is  reported  by  sight. 
It  is  true  that  what  we  have  observed  with  our  eyes  may 
be  set  forth  in  words,  and  thus  find  its  way  to  the  under- 
standing through  the  ears;  also  that  in  many  instances 
the  sense  of  touch  conveys  information  which  extends  our 
perceptions  in  many  important  ways;  but  science  rests  prac- 
tically on  sight,  and  on  the  insight  that  comes  from  the 
training  of  the  mind  which  the  eyes  make  possible. 

The  early  inquirers  had  no  resources  except  those 
their  bodies  afforded;  but  man  is  a  tool-making  creature, 
and  in  very  early  days  he  began  to  invent  instruments 
which  helped  him  in  inquiry.  The  earliest  deliberate 
study  was  of  the  stars.  Science  began  with  astronomy,  and 
the  first  instruments  which  men  contrived  ior  the  purpose 
of  investigation  were  astronomical.  In  the  beginning  of 
this  search  the  stars  were  studied  in  order  to  measure  the 
length  of  the  year,  and  also  for  the  reason  that  they  were 
supposed  in  some  way  to  control  the  fate  of  men.  So  far 
as  we  know,  the  first  pieces  of  apparatus  for  this  purpose 
were  invented  in  Egypt,  perhaps  about  four  thousand  years 


WAYS  AND  MEANS  OF  STUDYING  NATlTHE.        H 

before  the  Christian  era.  These  instruments  were  of  a 
simple  nature,  for  the  magnifying  glass  was  not  yet  con- 
trived, and  so  the  telescope  was  impossible.  They  con- 
sisted of  arrangements  of  straight  edges  and  divided 
circles,  so  that  the  observers,  by  sighting  along  the  instru- 
ments, could  in  a  rough  way  determine  the  changes  in 
distance  between  certain  stars,  or  the  height  of  the  sun 
above  the  horizon  at  the  various  seasons  of  the  year. 
It  is  likely  that  each  of  the  great  pyramids  of  Egypt  was  at 
first  used  as  an  observatory,  where  the  priests,  who  had 
some  knowledge  of  astronomy,  found  a  station  for  the  ap- 
paratus by  which  they  made  the  observations  that  ser\^d 
as  a  basis  for  casting  the  horoscope  of  the  king. 

In  the  progress  of  science  and  of  the  mechanical  inven- 
tion attending  its  growth,  a  great  number  of  inventions 
have  been  contrived  which  vastly  increase  our  vision  and 
add  inconceivably  to  the  precision  it  may  attain.  In  fact, 
something  like  as  much  skill  and  labour  has  been  given  to 
the  development  of  those  inventions  which  add  to  our 
learning  as  to  those  which  serve  an  immediate  economic 
end.  By  far  the  greatest  of  these  scientific  inventions  are 
those  which  depend  upon  the  lens.  By  combining  shaped 
bits  of  glass  so  as  to  control  the  direction  in  which  the  light 
waves  move  through  them,  naturalists  have  been  able  to 
create  the  telescope,  which  in  effect  may  bring  distant  ob- 
jects some  thousand  times  nearer  to  view  than  they  are  to 
the  naked  eye;  and  the  microscope,  which  so  enlarges  mi- 
nute objects  as  to  make  them  visible,  as  they  were  not 
before.  The  result  has  been  enormously  to  increase  our 
power  of  vision  when  applied  to  distant  or  to  small  objects. 
In  fact,  for  purposes  of  learning,  it  is  safe  to  say  that  those 
tools  have  altogether  changed  man's  relation  to  the  visible 
universe.  The  naked  eye  can  see  at  best  in  the  part  of 
the  heavens  visible  from  any  one  point  not  more  than 
thirty  thousand  stars.  With  the  telescope  somewhere  near 
a  hundred  million  are  brought  within  the  limits  of  vision. 
Without  the  help  of  the  microscope  an  object  a  thousandth 


12  OUTLINES  OF   THE  EARTH'S  HISTORY. 

of  an  inch  in  diameter  appears  as  a  mere  point,  the  exist- 
ence of  which  we  can  determine  only  under  favourable 
circumstances.  With  that  instrument  the  object  may  re- 
veal an  extended  and  complicated  structure  which  it  may 
require  a  vast  labour  for  the  observer  fully  to  explore. 

Next  in  importance  to  the  aid  of  vision  above  noted 
come  the  scientific  tools  which  are  used  in  weighing  and 
measuring.  These  balances  and  gauges  have  attained 
such  precision  that  intervals  so  small  as  to  be  quite  in- 
visible, and  weights  as  slight  as  a  ten-thousandth  of  a 
grain,  can  be  accurately  measured.  From  these  instru- 
ments have  come  all  those  precise  examinations  on  which 
the  accuracy  of  modern  science  intimately  depends.  All 
these  instruments  of  precision  are  the  inventions  of  modern 
days.  The  simplest  telescopes  were  made  only  about  two 
hundred  and  fifty  years  ago,  and  the  earlier  compound 
microscopes  at  a  yet  later  date.  Accurate  balances  and 
other  forms  of  gauges  of  space,  as  well  as  good  means  of 
dividing  time,  such  as  our  accurate  astronomical  clocks 
and  chronometers,  are  only  about  a  century  old.  The  in- 
struments have  made  science  accurate,  and  have  immensely 
extended  its  powers  in  nearly  all  the  fields  of  inquiry. 

Although  the  most  striking  modern  discoveries  are  in 
the  field  which  was  opened  to  us  by  the  lens  in  its  mani- 
fold applications,  it  is  in  the  chemist's  laboratory  that  we 
find  that  branch  of  science,  long  cultivated,  but  rapidly 
advanced  only  within  the  last  two  centuries,  which  has 
done  the  most  for  the  needs  of  man.  The  ancients  guessed 
that  the  substances  which  make  up  the  visible  world  were 
more  complicated  in  their  organization  than  they  appear 
to  our  vision.  They  even  suggested  the  great  truth  that 
matter  of  all  kinds  is  made  up  of  inconceivably  small  indi- 
visible bits  which  they  and  we  term  atoms.  It  is  likely 
that  in  the  classic  days  of  Greece  men  began  to  make  sim- 
ple experiments  of  a  chemical  nature.  A  century  or  two 
after  the  time  of  Mohammed,  the  Arabians  of  his  faith,  a 
people  who  had  acquired  Greek  science  from  the  libraries 


WAYS  AND  MEANS  OF  STUDYING  NATURE.       13 

which  their  conquests  gave  them,  conducted  extensive  ex- 
periments, and  named  a  good  many  familiar  chemical  prod- 
ucts, such  as  alcohol,  which  still  bears  its  Arabic  name. 

These  chemical  studies  were  continued  in  Europe  by 
the  alchemists,  a  name  also  of  Arabic  origin,  a  set  of  in- 
quirers who  were  to  a  great  extent  drawn  away  from  scien- 
tific studies  by  vain  though  unending  efforts  to  change 
the  baser  metals  into  gold  and  silver,  as  well  as  to  find  a 
compound  which  would  make  men  immortal  in  the  body. 
By  the  invention  of  the  accurate  balance,  and  by  patient 
weighing  of  the  matters  which  they  submitted  to  experi- 
ment, by  the  invention  of  hypotheses  or  guesses  at  truth, 
which  were  carefully  tested  by  experiment,  the  majestic 
science  of  modern  chemistry  has  come  forth  from  the  con- 
fused and  mystical  studies  of  the  alchemists.  We  have 
learned  to  know  that  there  are  seventy  or  more  primitive 
or  apparently  unchangeable  elements  which  make  up  the 
mass  of  this  world,  and  probably  constitute  all  the  celestial 
spheres,  and  that  these  elements  in  the  form  of  their  sepa- 
rate atoms  may  group  themselves  in  almost  inconceivably 
varied  combinations.  In  the  inanimate  realm  these  asso- 
ciations, composed  of  the  atoms  of  the  diifferent  substances, 
forming  what  are  termed  molecules,  are  generally  com- 
posed of  but  few  units.  Thus  carbonic-acid  gas,  as  it  is 
commonly  called,  is  made  up  of  an  aggregation  of  mole- 
cules, each  composed  of  one  atom  of  carbon  and  two  of 
oxygen;  water,  of  two  atoms  of  hydrogen  and  one  of 
oxygen;  ordinary  iron  oxide,  of  two  atoms  of  iron  and 
three  of  oxygen.  In  the  realm  of  organic  life,  however, 
these  combinations  become  vastly  more  complicated,  and 
with  each  of  them  the  properties  of  the  substance  thus 
produced  differ  from  all  others.  A  distinguished  chem- 
ist has  estimated  that  in  one  group  of  chemical  com- 
pounds, that  of  carbon,  it  would  be  possible  to  make  such 
an  array  of  substances  that  it  would  require  a  library  of 
many  thousand  ordinary  volumes  to  contain  their  names 
alone. 


14  OUTLINES  OF  THE  EARTH'S  HISTORY. 

It  is  characteristic  of  chemical  science  that  it  takes 
account  of  actions  which  are  almost  entirely  invisible.  No 
contrivances  have  been  or  are  likely  to  be  invented  which 
will  show  the  observer  what  takes  place  when  the  atoms 
of  any  substance  depart  from  their  previous  combination 
and  enter  on  new  arrangements.  We  only  know  that  un- 
der certain  conditions  the  old  atomic  associations  break  up, 
and  new  ones  are  formed.  But  though  the  processes  are 
hidden,  the  results  are  manifest  in  the  changes  which  are 
brought  about  upon  the  masses  of  material  which  are  sub- 
jected to  the  altering  conditions.  Gradually  the  chemists 
of  our  day  are  learning  to  build  up  in  their  laboratories 
more  and  more  complicated  compounds;  already  they  have 
succeeded  in  producing  many  of  the  materials  which  of 
old  could  only  be  obtained  by  extracting  them  from  plants. 
Thus  a  number  of  the  perfumes  of  flowers,  and  many  of 
the  dye-stuffs  which  a  century  ago  were  extracted  from 
vegetables,  and  were  then  supposed  to  be  only  obtainable 
in  that  way,  are  now  readily  manufactured.  In  time  it 
seems  likely  that  important  articles  of  food,  for  which 
we  now  depend  upon  the  seeds  of  plants,  may  be  directly 
built  up  from  the  mineral  kingdom.  Thus  the  result  of 
chemical  inquiry  has  been  not  only  to  show  us  much  of 
the  vast  realm  of  actions  which  go  on  in  the  earth,  but  to 
give  us  control  of  many  of  these  movements  so  that  we  may 
turn  them  to  the  needs  of  man. 

Animals  and  plants  were  at  an  early  day  very  naturally 
the  subjects  of  inquiry.  The  ancients  perceived  that  there 
were  differences  of  kind  among  these  creatures,  and  even 
in  Aristotle's  time  the  sciences  of  zoology  and  botany  had 
attained  the  point  where  there  were  considerable  treatises 
on  those  subjects.  It  was  not,  however,  until  a  little  more 
than  a  century  ago  that  men  began  accurately  to  describe 
and  classify  these  species  of  the  organic  world.  Since  the 
time  of  Linnaeus  the  growth  of  our  knowledge  has  gone 
forward  with  amazing  swiftness.  Within  a  century  we 
have  come  to  know  perhaps  a  hundred  times  as  much  con- 


WAYS  AND  MEANS  OF  STUDYING  NATURE.        15 

cerning  these  creatures  as  was  learned  in  all  the  earlier 
ages.  This  knowledge  is  divisible  into  two  main  branches: 
in  one  the  inquirers  have  taken  account  of  the  different 
species,  genera,  families,  orders,  and  classes  of  living  forms 
with  such  effect  that  they  have  shown  the  existence  at  the 
present  time  of  many  hundred  thousand  distinct  species, 
the  vast  assemblage  being  arranged  in  a  classification  which 
shows  something  as  to  the  relationship  which  the  forms 
bear  to  each  other,  and  furthermore  that  the  kinds  now 
living  have  not  been  long  in  existence,  but  that  at  each 
stage  in  the  history  of  the  earth  another  assemblage  of 
species  peopled  the  waters  and  the  lands. 

At  first  naturalists  concerned  themselves  only  with  the 
external  forms  of  living  creatures;  but  they  soon  came 
to  perceive  that  the  way  in  which  these  organisms  worked, 
their  physiology,  in  a  word,  afforded  matters  for  extended 
inquiry.  These  researches  have  developed  the  science  of 
physiology,  or  the  laws  of  bodily  action,  on  many  accounts 
the  most  modern  and  extensive  of  our  new  acquisitions  of 
natural  learning.  Through  these  studies  we  have  come 
to  know  something  of  the  laws  or  principles  by  which  life 
is  handed  on  from  generation  to  generation,  and  by  which 
the  gradations  of  structure  have  been  advanced  from  the 
simple  creatures  which  appear  like  bits  of  animated  jelly 
to  the  body  and  mind  of  man. 

The  greatest  contribution  which  modern  naturalists 
have  made  to  knowledge  concerns  the  origin  of  organic 
species.  The  students  of  a  century  ago  believed  that  all 
these  different  kinds  had  been  suddenly  created  either 
through  natural  law  or  by  the  immediate  will  of  God. 
We  now  know  that  from  the  beginning  of  organic  life  in 
the  remote  past  to  the  present  day  one  kind  of  animal  or 
plant  has  been  in  a  natural  and  essentially  gradual  way 
converted  into  the  species  which  was  to  be  its  successor, 
so  that  all  the  vast  and  complicated  assemblage  of  kinds 
which  now  exists  has  been  derived  by  a  process  of  change 
from  the  forms  which  in  earlier  ages  dwelt  upon  this 


1(5  OUTLINES  OF  THE  EARTH'S  HISTORY. 

planet.  The  exact  manner  in  which  these  alterations  were 
produced  is  not  yet  determined,  but  in  large  part  it  has 
evidently  been  brought  about  by  the  method  indicated  by 
Mr.  Darwin,  through  the  survival  of  the  fittest  individuals 
in  the  struggle  for  existence. 

Until  men  came  to  have  a  clear  conception  as  to  the 
spherical  form  of  the  earth,  it  was  impossible  for  them 
to  begin  any  intelligent  inquiries  concerning  its  structure 
or  history.  The  Greeks  knew  the  earth  to  be  a  sphere, 
but  this  knowledge  was  lost  among  the  early  Christian 
people,  and  it  was  not  until  about  four  hundred  years  ago 
that  men  again  came  to  see  that  they  dwelt  upon  a  globe. 
On  the  basis  of  this  understanding  the  science  of  geology, 
which  had  in  a  way  been  founded  by  the  Greeks,  was  re- 
vived. As  this  science  depends  upon  the  knowledge  v/hich 
we  have  gained  of  astronomy,  physics,  chemistry,  and 
biology,  all  of  which  branches  of  learning  have  to  be  used 
in  explaining  the  history  of  the  earth,  the  advance  v/hich 
has  been  made  has  been  relatively  slow.  Geology  as  a 
whole  is  the  least  perfectly  organized  of  all  the  divisions  of 
learning.  A  special  difficulty  peculiar  to  this  science  has 
also  served  to  hinder  its  development.  All  the  other 
branches  of  learning  deal  mainly,  if  not  altogether,  with 
the  conditions  of  Nature  as  they  now  exist.  In  this  alone 
is  it  necessary  at  every  step  to  take  account  of  actions 
which  have  been  performed  in  the  remote  past. 

It  is  an  easy  matter  for  the  students  of  to-day  to  im- 
agine that  the  earth  has  long  endured;  but  to  our  fore- 
fathers, who  were  educated  in  the  view  that  it  had  been 
brought  from  nothingness  into  existence  about  seven  thou- 
sand years  ago,  it  was  most  difficult  and  ■  for  a  time 
impossible  to  believe  in  its  real  antiquity.  Endeavouring, 
as  they  naturally  did,  to  account  for  all  the  wonderful  revo- 
lutions, the  history  of  which  is  written  in  the  pages  of 
the  great  stone  book,  the  early  geologists  supposed  this 
planet  to  have  been  the  seat  of  frequent  and  violent 
changes,  each  of  which  revolutionized  its  shape  and  de- 


WAYS  AND  MEANS  OF  STUDYING  NATURE.        17 

stroj^ed  its  living  tenants.  It  was  only  very  gradually  that 
they  became  convinced  that  a  hundred  million  years  or 
more  have  elapsed  since  the  dawn  of  life  on  the  earth, 
and  that  in  this  vast  period  the  march  of  events  has  been 
steadfast,  the  changes  taking  place  at  about  the  same  rate 
in  which  they  are  now  going  on.  As  yet  this  conception 
as  to  the  history  of  our  sphere  has  not  become  the  general 
property  of  the  people,  but  the  fact  of  it  is  recognised  by 
all  those  who  have  attentivel}'  studied  the  matter.  It  is 
now  as  well  ascertained  as  any  of  the  other  truths  which 
science  has  disclosed  to  us. 

It  is  instructive  to  note  the  historic  outlines  of  scien- 
tific development.  The  most  conspicuous  truth  which  this 
history  discloses  is  that  all  science  has  had  its  origin  and 
almost  all  its  development  among  the  peoples  belonging 
to  the  Aryan  race.  This  body  of  folk  appears  to  have 
taken  on  its  race  characteristics,  acquired  its  original  lan- 
guage, its  modes  of  action,  and  the  foundations  of  its 
religion  in  that  part  of  northern  Europe  which  is  about  the 
Baltic  Sea.  Thence  the  body  of  this  people  appear  to  have 
wandered  toward  central  Asia,  where  after  ages  of  pas- 
toral life  in  the  high  table  lands  and  mountains  of  their 
country  it  sent  forth  branches  to  India,  Asia  Minor  and 
Greece,  to  Persia,  and  to  western  Europe.  It  seems  ever 
to  have  been  a  characteristic  of  these  Arj^an  peoples  that 
they  had  an  extreme  love  for  Nature;  moreover,  they 
clearly  perceived  the  need  of  accounting  for  the  things 
that  happened  in  the  world  about  them.  In  general  they 
inclined  to  what  is  called  the  pantheistic  explanation  of 
the  universe.  They  believed  a  supreme  God  in  many  dif- 
ferent forms  to  be  embodied  in  all  the  things  they  saw. 
Even  their  own  minds  and  bodies  they  conceived  as  mani- 
festations of  this  supreme  power.  Among  the  Aryans  who 
came  to  dwell  in  Europe  and  along  the  eastern  Mediter- 
ranean this  method  of  explaining  Nature  was  in  time 
changed  to  one  in  which  humanlike  gods  were  supposed 
to  control  the  visible  and  invisible  worlds.     In  that  mar- 


18  OUTLINES  OF  THE  EARTH'S  HISTORY. 

vellous  centre  of  culture  which  was  developed  among  the 
Greeks  this  conception  of  humanlike  deities  was  in  time 
replaced  by  that  of  natural  law,  and  in  their  best  days 
the  Greeks  were  men  of  science  essentially  like  those  of 
to-day,  except  that  they  had  not  learned  by  experience  how 
important  it  was  to  criticise  their  theories  by  patiently  com- 
paring them  with  the  facts  which  they  sought  to  explain. 
The  last  of  the  important  Greek  men  of  science,  Sirabo, 
who  was  alive  when  Christ  was  born,  has  left  us  writings 
which  in  quality  are  essentially  like  many  of  the  able  works 
of  to-day.  But  for  the  interruption  in  the  development 
of  Greek  learning,  natural  science  would  probably  have 
been  fifteen  hundred  years  ahead  of  its  present  stage. 
This  interruption  came  in  two  ways.  In  one,  through 
the  conquest  of  Greece  and  the  destruction  of  its  intel- 
lectual life  by  the  Romans,  a  people  who  were  singularly 
incapable  of  appreciating  natural  science,  and  who  had 
no  other  interest  in  it  except  now  and  then  a  vacant  and 
unprofitable  curiosity  as  to  the  processes  of  the  natural 
world.  A  second  destructive  influence  came  through  the 
fact  that  Christianity,  in  its  energetic  protest  against  the 
sins  of  the  pagan  civilization,  absolutely  neglected  and  in 
a  way  despised  all  forms  of  science. 

The  early  indifference  of  Christians  to  natural  learning 
is  partly  to  be  explained  by  the  fact  that  their  religion  was 
developed  among  the  Hebrews,  a  people  remarkable  for 
their  lack  of  interest  in  the  scientific  aspects  of  Nature. 
To  them  it  was  a  sufficient  explanation  that  one  omnipo- 
tent God  ruled  all  things  at  his  will,  the  heavens  and  the 
earth  alike  being  held  in  the  hollow  of  his  hand. 

Finding  the  centre  of  its  development  among  the  Ro- 
mans, Christianity  came  mainly  into  the  control  of  a  peo- 
ple who,  as  we  have  before  remarked,  had.  no  scientific 
interest  in  the  natural  world.  This  condition  prolonged 
the  separation  of  our  faith  from  science  for  fifteen  hun- 
dred years  after  its  beginning.  In  this  time  the  records 
of  Greek  scientific  learning  mostly  disappeared.     The  writ- 


WAYS  AND  MEANS  OF  STUDYING  NATURE.   19 

ings  of  Aristotle  were  preserved  in  part  for  the  reason  that 
the  Church  adopted  many  of  his  views  concerning  ques- 
tions in  moral  philosophy  and  in  politics.  The  rest  of 
Greek  learning  was,  so  far  as  Europe  was  concerned,  quite 
neglected. 

A  large  part  of  Greek  science  which  has  come  down 
to  us  owes  its  preservation  to  a  very  singular  incident  in 
the  history  of  learning.  In  the  ninth  century,  after  the 
Arabs  had  been  converted  to  Mohammedanism,  and  on 
the  basis  of  that  faith  had  swiftly  organized  a  great  and 
cultivated  empire,  the  scholars  of  that  folk  became  deeply 
interested  in  the  remnants  of  Greek  learning  which  had 
survived  in  the  monastic  and  other  libraries  about  the 
eastern  Mediterranean.  So  greatly  did  they  prize  these 
records,  which  were  contemned  by  the  Christians,  that  it 
was  their  frequent  custom  to  weigh  the  old  manuscripts 
in  payment  against  the  coin  of  their  realm.  In  astronomy, 
mathematics,  chemistry,  and  geology  the  Arabian  students, 
building  on  the  ancient  foundations,  made  notable  and 
for  a  time  most  important  advances.  In  the  tenth  century 
of  our  era  they  seemed  fairly  in  the  way  to  do  for  science 
what  western  Europe  began  five  centuries  later  to  accom- 
plish. In  the  fourteenth  century  the  centre  of  Moham- 
medan strength  was  transferred  from  the  Arabians  to  the 
Turks,  from  a  people  naturally  given  to  learning  to  a  folk 
of  another  race,  who  despised  all  such  culture.  Thence- 
forth in  place  of  the  men  who  had  treasured  and  deciphered 
with  infinite  pains  all  the  records  of  earlier  learning,  the 
followers  of  Mohammed  zealously  destroyed  all  the  records 
of  the  olden  days.  Some  of  these  records,  however,  sur- 
vived among  the  Arabs  of  Spain,  and  others  were  preserved 
by  the  Christian  scholars  who  dwelt  in  Byzantium,  or 
Constantinople,  and  were  brought  into  western  Europe 
when  that  city  was  captured  by  the  Turks  in  the  fifteenth 
century. 

Already  the  advance  of  the  fine  arts  in  Italy  and  the 
general  tendency  toward  the  study  of  Nature,  such  as 


20  OUTLINES   OF  THE  EARTH'S  HISTORY. 

painting  and  sculpture  indicate,  had  made  a  beginning, 
or  rather  a  proper  field  for  a  beginning,  of  scientific  in- 
quiry. The  result  was  a  new  interest  in  Greek  learning 
in  all  its  branches,  and  a  very  rapid  awakening  of  the  scien- 
tific spirit.*  At  first  the  Roman  Church  made  no  opposition 
to  this  new  interest  which  developed  among  its  followers, 
but  in  the  course  of  a  few  years,  animated  with  the  fear  that 
science  would  lead  men  to  doubt  many  of  the  dogmas 
of  the  Church,  it  undertook  sternly  to  repress  the  work  of 
all  inquirers. 

The  conflict  between  those  of  the  Roman  faith  and 
the  men  of  science  continued  for  above  two  hundred  years. 
In  general,  the  part  which  the  Church  took  was  one  of 
remonstrance,  but  in  a  few  cases  the  spirit  of  fanaticism 
led  to  the  persecution  of  the  men  who  did  not  obey  its 
mandates  and  disavow  all  belief  in  the  new  opinions  which 
were  deemed  contrary  to  the  teachings  of  Scripture.  The 
last  instance  of  such  oppression  occurred  in  France  in  the 
year  1756,  when  the  great  Buff  on  was  required  to  recant 
certain  opinions  concerning  the  antiquity  of  the  earth 
which  he  had  published  in  his  work  on  Natural  History. 
This  he  promptly  did,  and  in  almost  servile  language  with- 
drew all  the  opinions  to  which  the  fathers  had  objected. 
A  like  conflict  between  the  followers  of  science  and  the 
clerical  authorities  occurred  in  Protestant  countries.  Al- 
though in  no  case  were  the  men  of  science  physically  tor- 
tured or  executed  for  their  opinions,  they  were  neverthe- 
less subjected  to  great  religious  and  social  pressure:  they 
were  almost  as  effectively  disciplined  as  were  those  who 
fell  under  the  ban  of  the  Roman  Church. 

Some  historians  have  criticised  the  action  of  the  clerical 
authorities  toward  science  as  if  the  evil  which  was  done 
had  been  performed  in  our  own  day.  It  should  be  remem- 
bered, however,  that  in  the  earlier  centuries  the  churches 
regarded  themselves  as  bound  to  protect  all  men  from  the 
dangers  of  heresy.  For  centuries  in  the  early  history  of 
Christianity  the  defenders  of  the  faith  had  been  engaged 


WAYS  AND  MEANS  OF  STUDYING  NATURE.       21 

in  a  life-and-deatli  struggle  with  paganism,  the  followers 
of  which  held  all  that  was  known  of  Nature.  Quite  natu- 
rally the  priestly  class  feared  that  the  revival  of  scien- 
tific inquiry  would  bring  with  it  the  evils  from  which  the 
world  had  suffered  in  pagan  times.  There  is,  no  doubt  that 
these  persecutions  of  science  were  done  under  what  seemed 
the  obligations  of  duty.  They  may  properly  be  explained 
particularly  by  men  of  science  as  one  of  the  symptoms  of 
development  in  the  day  in  which  they  were  done.  It  is 
well  for  those  who  harshly  criticise  the  relations  of  the 
Church  to  science  to  remember  that  in  our  own  country, 
about  two  centuries  ago,  among  the  most  enlightened  and 
religious  people  of  the  time,  Quakers  were  grievously  perse- 
cuted, and  witches  hanged,  all  in  the  most  dutiful  and  God- 
fearing way.  In  considering  these  relations  of  science  to 
our  faith,  the  matter  should  be  dealt  with  in  a  philosophical 
way,  and  with  a  sense  of  the  differences  between  our  own 
and  earlier  ages. 

To  the  student  of  the  relations  between  Christianity 
and  science  it  must  appear  doubtful  whether  the  criticism 
or  the  other  consequences  which  the  men  of  science  had 
to  meet  from  the  Church  was  harmful  to  their  work.  The 
early  naturalists,  like  the  Greeks  whom  they  followed,  were 
greatly  given  to  speculations  concerning  the  processes  of 
Nature,  which,  though  interesting,  were  unprofitable. 
They  also  showed  a  curious  tendency  to  mingle  their  scien- 
tific speculations  with  ancient  and  base  superstitions.  They 
were  often  given  to  the  absurdity  commonly  known  as  the 
"  black  art,"  or  witchcraft,  and  held  to  the  preposterous 
notions  of  the  astrologists.  Even  the  immortal  astronomer 
Kepler,  who  lived  in  the  sixteenth  century,  was  a  profes- 
sional astrologer,  and  still  held  to  the  notion  that  the  stars 
determined  the  destiny  of  men.  Many  other  of  the  famous 
inquirers  in  those  years  which  ushered  in  modern  science 
believed  in  witchcraft.  Thus  for  a  time  natural  learning 
was  in  a  way  associated  with  ancient  and  pernicious  beliefs 
which  the  Church  was  seeking  to  overthrow.     One  result 


22  OUTLINES  OF  THE  EARTH'S  HISTORY. 

of  the  clerical  opposition  to  the  advancement  of  science 
was  that  its  votaries  were  driven  to  prove  every  step  which 
led  to  their  conclusions.  They  were  forced  to  abandon  the 
loose  speculation  of  their  intellectual  guides,  the  Greeks, 
and  to  betake  themselves  to  observation.  Thus  a  part  of 
the  laborious  fact-gathering  habit  on  which  the  modern 
advance  of  science  has  absolutely  depended  was  due  to 
the  care  which  men  had  to  exercise  in  face  of  the  religious 
authorities. 

In  our  own  time,  in  the  latter  part  of  the  nineteenth 
century,  the  conflict  between  the  religious  authority  and 
the  men  of  science  has  practically  ceased.  Even  the  Eoman 
Church  permits  almost  everywhere  an  untrammelled  teach- 
ing of  the  established  learning  to  which  it  was  at  one  time 
opposed.  Men  have  come  to  see  that  all  truth  is  accord- 
ant, and  that  religion  has  nothing  to  fear  from  the  faithful 
and  devoted  study  of  Nature. 

The  advance  of  science  in  general  in  modern  times 
has  been  greatly  due  to  the  development  of  mechanical 
inventions.  Among  the  ancients,  the  tools  which  served 
in  the  arts  were  few  in  number,  and  these  of  exceeding 
simplicity.  So  far  as  we  can  ascertain,  in  the  five  hundred 
years  during  which  the  Greeks  were  in  their  intellectual 
vigour,  not  more  than  half  a  dozen  new  machines  were 
invented,  and  these  were  exceedingly  simple.  The  fact 
seems  to  be  that  a  talent  for  mechanical  invention  is  mainly 
limited  to  the  peoples  of  France,  Germany,  and  of  the 
English-speaking  folk.  The  first  advances  in  these  con- 
trivances were  made  in  those  countries,  and  all  our  con- 
siderable gains  have  come  from  their  people.-  Thus,  while 
the  spirit  of  science  in  general  is  clearly  limited  to  the 
Aryan  folk,  that  particular  part  of  the  motive  which  leads 
to  the  invention  of  tools  is  restricted  to  western  and  north- 
ern Europe,  to  the  people  to  whom  we  give  the  name  of 
Teutonic. 

Mechanical  inventions  have  aided  the  development  of 
our  sciences  in  several  ways.  They  have  furnished  inquirers 


WAYS  AND  MEANS  OF  STUDYING  NATURE.       23 

with  instruments  of  precision;  they  have  helped  to  develop 
accuracy  of  observation;  best  of  all,  they  have  served  ever 
to  bring  before  the  attention  of  men  a  spectacle  of  the 
conditions  in  Nature  which  we  term  cause  and  effect.  The 
influence  of  these  inventions  on  the  development  of  learn- 
ing has  been  particularly  great  where  the  machines,  such  as 
our  wind  and  water  mills,  and  our  steam  engine,  make  usa 
of  the  forces  of  Nature,  subjugating  them  to  the  needs  of 
man.  Such  instruments  give  an  unending  illustration 
as  to  the  presence  in  Nature  of  energy.  They  have  helped 
men  to  understand  that  the  machinery  of  the  universe  is 
propelled  l)y  the  unending  application  of  power.  It  was, 
in  fact,  through  such  machines  that  men  of  science  first 
came  to  understand  that  energy,  manifested  in  the  natural 
forces,  is  something  that  eternally  endures;  that  we  may 
change  its  form  in  our  arts  as  its  form  is  changed  in  the 
operations  of  Nature,  but  the  power  endures  forever. 

It  is  interesting  to  note  that  the  first  observation  which 
led  to  this  most  important  scientific  conclusion  that  energy 
is  indestructible  however  much  it  may  change  its  form, 
was  made  by  an  American,  Benjamin  Thompson,  who  left 
this  country  at  the  time  of  the  Revolution,  and  after  a 
jurious  life  became  the  executive  officer,  and  in  effect  king, 
jf  Bavaria.  While  engaged  in  superintending  the  manu- 
facture of  cannon,  he  observed  that  in  boring  out  the  barrel 
of  the  gun  an  amount  of  heat  was  produced  which  evapo- 
rated a  certain  amount  of  water.  He  therefore  concluded 
that  the  energy  required  to  do  the  boring  of  the  metal 
passed  into  the  state  of  heat,  and  thus  only  changed  its 
state,  in  no  wise  disappearing  from  the  earth.  Other  stu- 
dents pursuing  the  same  line  of  inquiry  have  clearly 
demonstrated  what  is  called  the  law  of  the  conservation 
of  energy,  which  more  than  anything  has  helped  us  to 
understand  the  large  operations  of  Nature.  Through  these 
studies  we  have  come  to  see  that,  while  the  universe  is  a 
place  of  ceaseless  change,  the  quantities  of  energy  and  of 
matter  remain  unaltered. 
3 


24:  OUTLIKES  OF  THE  EAETirs  HISTORY. 

The  foregoing  brief  sketch,  which  sets  forth  some  of 
the  important  conditions  which  have  affected  the  develop- 
ment of  science,  may  in  a  way  serve  to  show  the  student 
how  he  can  himself  become  an  interpreter  of  Nature.  The 
evidence  indicates  that  the  people  of  our  race  have  been 
in  a  way  chosen  among  all  the  varieties  of  mankind  to  lead 
in  this  great  task  of  comprehending  the  visible  universe. 
The  facts,  moreover,  show  that  discovery  usually  begins 
with  the  interest  which  men  feel  in  the  world  immediately 
about  them,  or  which  is  presented  to  their  senses  in  a  daily 
spectacle.  Thus  Benjamin  Franklin,  in  the  midst  of  a 
busy  life,  became  deeply  interested  in  the  phenomena  of 
lightning,  and  by  a  very  simple  experiment  proved  that 
this  wonder  of  the  air  was  due  to  electrical  action  such  as 
we  may  arouse  by  rubbing  a  stick  of  sealing-wax  or  a  piece 
of  amber  with  a  cloth.  All  discoveries,  in  a  word,  have 
had  their  necessary  beginnings  in  an  interest  in  the  facts 
which  daily  experience  discloses.  This  desire  to  know 
something  more  than  the  first  sight  exhibits  concerning 
the  actions  in  the  world  about  us  is  native  in  every  human 
soul — at  least,  in  all  those  who  are  born  with  the  heritage 
of  our  race.  It  is  commonly  strong  in  childhood;  if  cul- 
tivated by  use,  it  will  grow  throughout  a  lifetime,  and, 
like  other  faculties,  becomes  the  stronger  and  more  effect- 
ive by  the  exertions  which  it  inspires.  It  is  therefore  most 
important  that  every  one  should  obey  this  instinctive  com- 
mand to  inquiry,  and  organize  his  life  and  work  so  that 
he  may  not  lose  but  gain  more  and  more  as  time  goes  on 
of  this  noble  capacity  to  interrogate  and  understand  the 
world  about  him. 

It  is  best  that  all  study  of  Nature  should  begin  not  in 
laboratories,  nor  with  the  things  which  are  remote  from 
us,  but  in  the  field  of  Nature  which  is  immediately  about 
us.  The  student,  even  if  he  dwell  in  the  unfavourable 
conditions  of  a  great  city,  is  surrounded  by  the  world  which 
has  yielded  immeasurable  riches  in  the  way  of  learning, 
which  he  can  appropriate  by  a  little  study.    He  can  readily 


WAYS  AND  MEANS  OF  STUDYING  NATURE.        25 

come  to  know  something  of  the  movements  of  the  air;  the 
buildings  will  give  him  access  to  a  great  many  different 
kinds  of  stone;  the  smallest  park,  a  little  garden,  or  even 
a  few  potted  plants  and  captive  animals,  may  tell  him  much 
concerning  the  forms  and  actions  of  living  beings.  By 
studying  in  this  way  he  can  come  to  know  something  of 
the  differences  between  things  and  their  relations  to  each 
other.  He  will  thus  have  a  standard  by  which  he  can 
measure  and  make  familiar  the  body  of  learning  concerning 
Nature  which  he  may  find  in  books.  From  printed  pages 
alone,  however  well  they  be  written,  he  can  never  hope 
to  catch  the  spirit  that  animates  the  real  inquirer,  the 
true  lover  of  Nature. 

On  many  accounts  the  most  attractive  way  of  begin- 
ning to  form  the  habit  of  the  naturalist  is  by  the  study 
of  living  animals  and  plants.  To  all  of  us  life  adds  inter- 
est, and  growth  has  a  charm.  Therefore  it  is  well  for  the 
student  to  start  on  the  way  of  inquiry  by  watching  the 
actions  of  birds  and  insects  or  by  rearing  plants.  It  is 
fortunate  if  he  can  do  both  these  agreeable  things.  When 
the  habit  of  taking  an  account  of  that  most  important  part 
of  the  world  which  is  immediately  about  him  has  been 
developed  in  the  student,  he  may  profitably  proceed  to 
acquire  the  knowledge  of  the  invisible  universe  which  has 
been  gathered  by  the  host  of  inquirers  of  his  race.  How- 
ever far  he  journeys,  he  should  return  to  the  home  world 
that  lies  immediately  and  familiarly  about  him,  for  there 
alone  can  he  acquire  and  preserve  that  personal  acquaint- 
ance with  things  which  is  at  once  the  inspiration  and  the 
test  of  all  knowledge. 

Along  with  this  study  of  the  familiar  objects  about 
us  the  student  may  well  combine  some  reading  which  may 
serve  to  show  him  how  others  have  been  successful  in  thus 
dealing  with  Nature  at  first  hand.  For  this  purpose  there 
are,  unfortunately,  but  few  works  which  are  well  calculated 
to  serve  the  needs  of  the  beginner.  Perhaps  the  best  natu- 
ralist book,  though  its  form  is  somewhat  ancient,  is  White's 


26  OUTLINES  OF  THE  EARTH'S  HISTORY. 

Natural  History  of  Selborne.  Hugh  Miller's^  works,  par- 
ticularly his  Old  Eed  Sandstone  and  My  Schools  and 
Schoolmasters,  show  well  how  a  man  may  become  a  natu- 
ralist under  difficulties.  Sir  John  Lubbock's  studies  on 
Wasps,  and  Darwin's  work  on  Animals  and  Plants  under 
Domestication  are  also  admirable  to  show  how  observation 
should  be  made.  Dr.  Asa  Gray's  little  treatise  on  How 
Plants  Grow  will  also  be  useful  to  the  beginner  who  wishes 
to  approach  botany  from  its  most  attractive  side — that  oi 
the  development  of  the  creature  from  the  seed  to  seed. 

There  is  another  kind  of  training  which  every  beginnei 
in  the  art  of  observing  Nature  should  obtain,  and  which 
many  naturalists  of  repute  would  do  well  to  give  them- 
selves— namely,  an  education  in  what  we  may  call  the  art 
of  distance  and  geographical  forms.  With  the  primitive 
savage  the  capacity  to  remember  and  to  picture  to  the 
eye  the  shape  of  a  country  which  he  knows  is  native  and 
instinctive.  Accustomed  to  range  the  woods,  and  to  trust 
to  his  recollection  to  guide  him  through  the  wilderne::S 
to  his  home,  the  primitive  man  develops  an  important  art 
which  among  civilized  people  is  generally  dormant.  In 
fact,  in  our  well-trodden  ways  people  may  go  for  many 
generations  without  ever  being  called  upon  to  use  this 
natural  sense  of  geography.  The  easiest  way  to  cultivate 
the  geographic  sense  is  by  practising  the  art  of  making 
sketch  maps.  This  the  student,  however  untrained,  can 
readily  do  by  taking  first  his  own  dwelling  house,  on  which 
he  should  practise  until  he  can  readily  from  memory  make 
a  tolerably  correct  and  proportional  plan  of  all  its  rooms. 
Then  on  a  smaller  scale  he  should  begin  to  make  also  from 
recollection  a  map  showing  the  distribution  of  the  roads, 
streams,  and  hills  with  which  his  daily  life  makes  him 
familiar.  From  time  to  time  this  work  from  memory 
should  be  compared  with  the  facts.  At  first  the  record 
will  be  found  to  be  very  poor,  but  with  a  few  months  of 
occasional  endeavour  the  observer  will  find  that  his  mind 
t^k^s  account  of  geographic  features  in  a  way  it  did  not 


WAYS  AND  MEANS  OF  STUDYING  NATURE.   27 

before,  and,  moreover,  that  his  mind  becomes  enriched 
with  impressions  of  the  country  which  are  clear  and  dis- 
tinct, in  place  of  the  shadowy  recollections  which  he  at 
first  possessed. 

When  the  student  has  attained  the  point  where,  after 
walking  or  riding  over  a  country,  he  can  readily  recall  its 
phj'sical  features  of  the  simpler  sort,  he  will  find  it  profit- 
able to  undertake  the  method  of  mapping  with  contour 
lines — that  is,  by  pencilling  in  indications  to  show  the 
exact  shape  of  the  elevations  and  depressions.  The  prin- 
ciple of  contour  lines  is  that  each  of  them  represents  where 
water  would  come  against  the  slope  if  the  area  were  sunk 
step  by  step  below  the  sea  level — in  other  words,  each  con- 
tour line  marks  the  intersection  of  a  horizontal  plane  with 
the  elevation  of  the  country.  Practice  on  this  somewhat 
difficult  task  w^ill  soon  give  the  student  some  idea  as  to  the 
complication  of  the  surface  of  a  region,  and  afford  him  the 
basis  for  a  better  understanding  of  what  geography  means 
than  all  the  reading  he  can  do  will  effect.  It  is  most  de- 
sirable that  training  such  as  has  been  described  should  be 
a  part  of  our  ordinary  school  education. 

Very  few  people  have  clear  ideas  of  distances.  Even 
the  men  whose  trade  requires  some  such  knowledge  are 
often  without  that  which  a  litle  training  could  give  them. 
Without  some  capacity  in  this  direction,  the  student  is 
always  at  a  disadvantage  in  his  contact  with  Nature.  He 
can  not  make  a  record  of  what  he  sees  as  long  as  the  ele- 
ment of  horizontal  and  vertical  distance  is  not  clearly  in 
mind.  To  attain  this  end  the  student  should  begin  by 
pacing  some  length  of  road  where  the  distances  are  well 
known.  In  this  way  he  will  learn  the  length  of  his  step, 
which  with  a  grown  man  generally  ranges  between  two  and 
a  half  and  three  feet.  Learning  the  average  length  of  his 
stride  by  frequent  counting,  it  is  easy  to  repeat  the  trial 
until  one  can  almost  unconsciously  keep  the  count  as  he 
walks.  Properly  to  secure  the  training  of  this  sort  the 
observer  should  first  attentively  look  across  the  distance 


28  OUTLINES  OF  THE  EARTH'S  HISTORY. 

which  is  to  be  determined.  He  should  notice  how  houses, 
fences,  people,  and  trees  appear  at  that  distance.  He  will 
quickly  perceive  that  each  hundred  feet  of  additional  in- 
terval somewhat  changes  their  aspect.  In  training  soldiers 
to  measure  with  the  eye  the  distances  which  they  have  to 
know  in  order  effectively  to  use  the  modern  weapons  of 
war,  a  common  device  is  to  take  a  squad  of  men,  or  some- 
times a  company,  under  the  command  of  an  officer,  who 
halts  one  man  at  each  hundred  yards  until  the  detachment 
is  strung  out  with  that  interval  as  far  as  the  eye  can  see 
them.  The  men  then  walk  to  and  fro  so  that  the  troops 
who  are  watching  them  may  note  the  effects  of  increased 
distance  on  their  appearance,  whether  standing  or  in  mo- 
tion. At  three  thousand  yards  a  man  appears  as  a  mere 
dot,  which  is  not  readily  distinguishable.  Schoolboys  may 
find  this  experiment  amusing  and  instructive. 

After  the  student  has  gained,  as  he  readily  may,  some 
sense  of  the  divisions  of  distance  within  the  range  of 
ordinary  vision,  he  should  try  to  form  some  notion  of 
greater  intervals,  as  of  ten,  a  hundred,  and  perhaps  a 
thousand  miles.  The  task  becomes  more  difficult  as  the 
length  of  the  line  increases,  but  most  persons  can  witli  a 
little  address  manage  to  bring  before  their  eyes  a  tolerably 
clear  image  of  a  hundred  miles  of  distance  by  looking  from 
some  elevation  which  commands  a  great  landscape.  It  is 
doubtful,  however,  whether  the  best-trained  man  can  get 
any  clear  notion  of  a  thousand  miles — that  is,  can  present 
it  to  himself  in  imagination  as  he  may  readily  do  with 
shorter  intervals. 

The  most  difficult  part  of  the  general  education  which 
the  student  has  to  give  himself  is  begun  when  he  under- 
takes to  picture  long  intervals  of  time.  Space  we  have 
opportunities  to  measure,  and  we  come  in  a  way  to  appre- 
ciate it,  but  the  longest  lived  of  men  experiences  at  most 
a  century  of  life,  and  this  is  too  small  a  measure  to  give 
any  notion  as  to  the  duration  of  such  great  events  as  are 
involved  in  the  history  of  the  earth,  where  the  periods 


WAYS  AND  MEANS  OF  STUDYING  NATURE.       29 

are  to  be  reckoned  by  the  millions  of  years.  The  only 
way  in  which  we  can  get  any  aid  in  picturing  to  ourselves 
great  lapses  of  time  is  by  expressing  them  in  units  of  dis- 
tance. Let  a  student  walk  away  on  a  straight  road  for  the 
distance  of  a  mile;  let  him  call  each  step  a  year;  when  he 
has  won  the  first  milestone,  he  may  consider  that  he  has 
gone  backward  in  time  to  the  period  of  Christ's  birth.  Two 
miles  more  will  take  him  to  the  station  which  will  repre- 
sent the  age  when  the  oldest  pyramids  were  built.  He  is 
still,  however,  in  .the  later  days  of  man's  history  on  this 
planet.  To  attain  on  the  scale  the  time  when  man  began, 
he  might  well  have  to  walk  fifty  miles  away,  while  a  jour- 
ney which  would  thus  by  successive  steps  describe  the  years 
of  the  earth's  history  since  life  appeared  upon  its  surface 
would  probably  require  him  to  circle  the  earth  at  least 
four  times.  We  may  accejjt  it  as  impossible  for  any  one  to 
deal  with  such  vast  durations  save  with  figures  which  are 
never  really  comprehended.  It  is  well,  however,  to  enlarge 
our  view  as  to  the  age  of  the  earth  by  such  efforts  as  have 
just  been  indicated. 

When  we  go  beyond  the  earth  into  the  realm  of  the 
stars  all  efforts  toward  understanding  the  ranges  of  space 
or  the  durations  of  time  are  quite  beyond  the  efforts  of 
man*  Even  the  distance  of  about  two  hundred  and  forty 
thousand  miles  which  separates  us  from  the  moon  can  not 
bf  grasped  by  even  the  greater  minds.  No  human  intelli- 
gence, however  cultivated,  can  conceive  the  distance  of 
jlbout  ninety-five  million  miles  which  separates  us  from 
the  sun.  In  the  celestial  realm  we  can  only  deal  with  re- 
lations of  space  and  time  in  a  general  and  comparative 
way.  We  can  state  the  distances  if  we  please  in  millions 
of  miles,  or  we  can  reckon  the  ampler  spaces  by  using  the 
interval  which  separates  the  earth  from  the  sun  as  we  do 
a  foot  rule  in  our  ordinary  work,  but  the  depths  of  the 
starry  spaces  can  only  be  sounded  by  the  winged  imagi- 
nation. 

Although  the  student  has  been  advised  to  begin  his 


30  OUTLINES  OF  THE  EARTH'S  HISTORY. 

studies  of  Nature  on  the  field  whereon  he  dwells,  making 
that  study  the  basis  of  his  most  valuable  communications 
with  Nature,  it  is  desirable  that  he  should  at  the  same  time 
gain  some  idea  as  to  the  range  and  scope  of  our  knowledge 
concerning  the  visible  universe.  As  an  aid  toward  this  end 
the  following  chapters  of  this  book  will  give  a  very  brief 
survey  of  some  of  the  most  important  truths  concerning 
the  heavens  and  the  earth  which  have  rewarded  the  studies 
of  scientific  men.  Of  remoter  things,  such  as  the  bodies  in 
the  stellar  spaces,  the  account  will  be  brief,  for  that  which 
is  known  and  important  to  the  general  student  can  be 
briefiy  told.  So,  too,  of  the  earlier  ages  of  the  earth's  his- 
tory, although  a  vast  deal  is  known,  the  greater  part  of  the 
knowledge  is  of  interest  and  value  mainly  to  geologists 
who  cultivate  that  field.  That  which  is  most  striking  and 
most  important  to  the  mass  of  mankind  is  to  be  found  in 
the  existing  state  of  our  earth,  the  conditions  which  make 
it  a  fit  abode  for  our  kind,  and  replete  with  lessons  which 
he  may  study  with  his  own  eyes  without  having  to  travel 
the  difficult  paths  of  the  higher  sciences. 

Although  physiography  necessarily  takes  some  account 
of  the  things  which  have  been,  even  in  the  remote  past, 
and  this  for  the  reason  that  everything  in  this  day  of  the 
world  depends  on  the  events  of  earlier  days,  the  accent  of 
its  teaching  is  on  the  immediate,  visible,  as  we  may  say, 
living  world,  which  is  a  part  of  the  life  of  all  its  inhabit- 
ants. 


CHAPTER  III. 

THE   STELLAR   REALM. 

Even  before  men  came  to  take  any  careful  account  of 
the  Nature  immediately  about  them  they  began  to  con- 
jecture and  in  a  way  to  inquire  concerning  the  stars  and 
the  other  heavenly  bodies.  It  is  difficult  for  us  to  imagine 
how  hard  it  was  for  students  to  gain  any  adequate  idea  of 
what  those  lights  in  the  sky  really  are.  At  first  men 
imagined  the  celestial  bodies  to  be,  as  they  seemed,  small 
objects  not  very  far  away.  Among  the  Greeks  the  view 
grew  up  that  the  heavens  were  formed  of  crystal  spheres 
in  which  the  lights  were  placed,  much  as  lanterns  may  be 
hung  upon  a  ceiling.  These  spheres  were  conceived  to 
be  one  above  the  other;  the  planets  were  on  the  lower  of 
them,  and  the  fixed  stars  on  the  higher,  the  several  crystal 
roofs  revolving  about  the  earth.  So  long  as  the  earth  was 
supposed  to  be  a  flat  and  limitless  expanse,  forming  the 
centre  of  the  universe,  it  was  impossible  for  the  students 
of  the  heavens  to  attain  any  more  rational  view  as  to  their 
plan. 

The  fact  that  the  earth  was  globular  in  form  was  un- 
derstood by  the  Greek  men  of  science.  They  may,  indeed, 
have  derived  the  opinion  from  the  Egyptian  philosophers. 
The  discovery  rested  upon  the  readily  observed  fact  that 
on  a  given  day  the  shadow  of  objects  of  a  certain  height 
was  longer  in  high  latitude  than  in  low.  Within  the  trop'cs, 
when  the  sun  was  vertical,  there  would  be  no  shadow,  while 
as  far  north  as  Athens  it  would  be  of  considerable  length. 
The  conclusion  that  the  earth  was  a  sphere  appears  to  have 

31 


32  OUTLINES  OF  THE  EARTH'S  HISTORY. 

been  the  first  large  discovery  made  by  our  race.  It  was, 
indeed,  one  of  tlie  most  important  intellectual  acquisitions 
of  man. 

Understanding  the  globular  form  of  the  earth,  the  next 
and  most  natural  step  was  to  learn  that  the  earth  was  not 
the  centre  of  the  planetary  system,  much  less  of  the  uni- 
verse, but  that  that  centre  was  the  sun,  around  which  the 
earth  and  the  other  planets  revolved.  The  Greeks  appear 
to  have  had  some  idea  that  this  was  the  case,  and  their 
spirit  of  inquiry  would  probably  have  led  them  to  the  whole 
truth  but  for  the  overthrow  of  their  thought  by  the  Eoman 
conquest  and  the  spread  of  Christianity.  It  was  therefore 
not  until  after  the  revival  of  learning  that  astronomers 
won  their  way  to  our  modern  understanding  concerning 
the  relation  of  the  planets  to  the  sun.  With  Galileo  this 
opinion  was  affirmed.  Although  for  a  time  the  Church, 
resting  its  opposition  on  the  interpretation  of  certain  pas- 
sages of  Scripture,  resisted  this  view,  and  even  punished 
the  men  who  held  it,  it  steadfastly  made  its  way,  and  for 
more  than  two  centuries  has  been  the  foundation  of  all 
the  great  discoveries  in  the  stellar  realm.  Yet  long  after 
the  fact  that  the  sun  was  the  centre  of  the  solar  system 
was  well  established  no  one  understood  why  the  planets 
should  move  in  their  ceaseless,  orderly  procession  around 
the  central  mass.  To  Newton  we  owe  the  studies  on  the 
law  of  gravitation  which  brought  us  to  our  present  large 
conception  as  to  the  origin  of  this  order.  Starting  with 
the  view  that  bodies  attracted  each  other  in  proportion  to 
their  w^eight,  and  in  diminishing  proportion  as  they  are 
removed  from  each  other,  Newton  proceeded  by  most 
laborious  studies  to  criticise  this  view,  and  in  the  end 
definitely  proved  it  by  finding  that  the  motions  of  the 
moon  about  the  earth,  as  well  as  the  paths  of  the  planets, 
exactly  agreed  with  the  supposition. 

The  last  great  path-breaking  discovery  which  has 
helped  us  in  our  understanding  of  the  stars  was  made  by 
Fraunhofer  and  other  physicists,  who  showed  us  that  sub- 


THE  STELLAR  REALM.  33 

stances  when  in  a  heated,  gaseous,  or  vaporous  state  pro- 
duced, in  a  way  which  it  is  not  easy  to  explain  in  a  work 
such  as  this,  certain  dark  lines  in  the  spectrum,  or  streak 
of  divided  light  which  we  may  make  by  means  of  a  glass 
prism,  or,  as  in  the  rainbow,  by  drops  of  water.  Carefully 
studying  these  very  numerous  line's,  those  naturalists  found 
that  they  could  with  singular  accuracy  determine  what  sub- 
stances there  were  in  the  flame  which  gave  the  light.  So 
accurate  is  this  determination  that  it  has  been  made  to 
serve  in  certain  arts  where  there  is  no  better  means  of 
ascertaining  the  conditions  of  a  flaming  substance  except 
by  the  lines  which  its  light  exhibits  under  this  kind  of 
analysis.  Thus,  in  the  manufacture  of  iron  by  what  is 
called  the  Bessemer  process,  it  has  been  found  very  con- 
venient to  judge  as  to  the  state  of  the  molten  metal  by 
such  an  analysis  of  the  flame  which  comes  forth  from  it. 

No  sooner  was  the  spectroscope  invented  than  astrono- 
mers hastened  by  its  aid  to  explore  the  chemical  constitu- 
tion of  the  sun.  These  studies  have  made  it  plain  that 
the  light  of  our  solar  centre  comes  forth  from  an  atmos- 
phere composed  of  highly  heated  substances,  all  of  which 
are  known  among  the  materials  forming  the  earth.  Al- 
though for  various  reasons  we  have  not  been  able  to  recog- 
nise in  the  sun  all  tlie  elements  which  are  found  in  our 
sphere,  it  is  certain  tliat  in  general  the  two  bodies  are 
alike  in  composition.  An  extension  of  the  same  method 
of  inquiry  to  the  fixed  stars  was  gradually  though  with 
difficulty  attained,  and  we  now  know  that  many  of  the 
elements  common  to  the  sun  and  earth  exist  in  those  dis- 
tant spheres.  Still  further,  this  method  of  inquiry  has 
shown  us,  in  a  way  which  it  is  not  worth  while  here  to 
describe,  that  among  these  remoter  suns  there  are  many 
aggregations  of  matter  which  are  not  consolidated  as  are 
the  spheres  of  our  own  solar  system,  but  remain  in  the 
gaseous  state,  receiving  the  name  of  nebulae. 

Along  with  the  growth  of  observational  astronomy 
which  has  taken  place  since  the  discoveries  of  Galileo, 


34  OUTLINES  OF  THE  EARTH'S  HISTORY. 

there  lias  been  developed  a  view  concerning  the  physical 
history  of  the  stellar  world,  known  as  the  nebular  hypothe- 
.  sis,  which,  though  not  yet  fully  proved,  is  believed  by  most 
astronomers  and  physicists  to  give  us  a  tolerably  correct 
notion  as  to  the  way  in  which  the  heavenly  spheres  were 
formed  from  an  earlier  condition  of  matter.  This  majes- 
tic conception  was  first  advanced,  in  modern  times  at  least, 
by  the  German  philosopher  Immanuel  Kant.  It  was  de- 
veloped by  the  French  astronomer  Laplace,  and  is  often 
known  by  his  name.  The  essence  of  this  view  rests  upon 
the  fact  previously  noted  that  in  the  realm  of  the  fixed  stars 
there  are  many  faintly  shining  aggregations  of  matter 
which  are  evidently  not  solid  after  the  manner  of  the 
bodies  in  our  solar  system,  but  are  in  the  state  where  their 
substances  are  in  the  condition  of  dustlike  particles,  as  are 
the  bits  of  carbon  in  flame  or  the  elements  which  compose 
the  atmosphere.  The  view  held  by  Laplace  was  to  the 
effect  that  not  only  our  own  solar  system,  but  the  centres 
of  all  the  other  similar  systems,  the  fixed  stars,  were  origi- 
nally in  this  gaseous  state,  the  material  being  disseminated 
throughout  all  parts  of  the  heavenly  realm,  or  at  least  in 
that  portion  of  the  universe  of  which  we  are  permitted 
to  know  something.  In  this  ancient  state  of  matter  we 
have  to  suppose  that  the  particles  of  it  were  more  sepa- 
rated from  each  other  than  are  the  atoms  of  the  atmos- 
pheric gases  in  the  most  perfect  vacuum  which  we  can 
produce  with  the  air-pump.  Still  we  have  to  suppose  that 
each  of  these  particles  attract  the  other  in  the  gravitative 
way,  as  in  the  present  state  of  the  universe  they  inevi- 
tably do. 

Under  the  influence  of  the  gravitative  -  attraction  the 
materials  of  this  realm  of  vapour  inevitably  tended  to  fall 
in  toAvard  the  centre.  If  the  process  had  been  perfectly 
simple,  the  result  would  have  been  the  formation  of  one 
vast  mass,  including  all  the  matter  which  was  in  the  origi- 
nal body.  In  some  way,  no  one  has  yet  been  able  to  make 
a  reasonable  suggestion  of  just  how,  there  were  developed 


THE  STELLAR  REALM.  35 

in  the  process  of  concentration  a  great  many  separate  cen- 
tres of  aggregation,  each  of  which  became  the  begin- 
ning of  a  solar  system.  The  student  may  form  some 
idea  of  how  readily  local  centres  may  be  produced  in  ma- 
terials disseminated  in  the  vaporous  state  by  watch- 
ing how  fog  or  the  thin,  even  misty  clouds  of  the  sun- 
rise often  gather  into  the  separate  shapes  which  make 
what  we  term  a  "  mackerel "  sky.  It  is  difficult  to 
imagine  what  makes  centres  of  attraction,  but  we  read- 
ily perceive  by  this  instance  how  they  might  have  oc- 
curred. 

When  the  materials  of  each  solar  system  were  thus  set 
apart  from  the  original  mass  of  star  dust  or  vapour,  they 
began  an  independent  development  which  led  step  by  step, 
in  the  case  of  our  own  solar  system  at  least,  and  presumably 
also  in  the  case  of  the  other  suns,  the  fixed  stars,  to  the 
formation  of  planets  and  their  moons  or  satellites,  all  mov- 
ing around  the  central  sun.  At  this  stage  of  the  explana- 
tion the  nebular  hypothesis  is  more  difficult  to  conceive 
than  in  the  parts  of  it  which  have  already  been  described, 
for  we  have  now  to  understand  how  the  planets  and  satel- 
lites had  their  matter  separated  from  each  other  and  from 
the  solar  centre,  and  why  they  came  to  revolve  around  that 
central  body.  These  problems  are  best  understood  by  not- 
ing some  familiar  instances  connected  with  the  movement 
of  fluids  and  gases  toward  a  centre.  First  let  us  take  the 
case  of  a  basin  in  which  the  water  is  allowed  to  flow  out 
through  a  hole  in  its  centre.  When  we  lift  the  stopper 
the  fluid  for  a  moment  falls  straight  down  through  the 
opening.  Very  quickly,  however,  all  the  particles  of  the 
water  start  to  move  toward  the  centre,  and  almost  at  once 
the  mass  begins  to  whirl  round  with  such  speed  that,  al- 
though it  is  working  toward  the  middle,  it  is  by  its  move- 
ment pushed  away  from  the  centre  and  forms  a  conical 
depression.  As  often  as  we  try  the  experiment,  the  effect 
is  always  the  same.  AVe  thus  see  that  there  is  some  prin- 
ciple which  makes  particles  of  fluid  that  tend  toward  a 


36  OUTLINES  OF  THE  EARTH'S  HISTORY. 

centre  fail  directly  to  attain  it,  but  win  their  way  thereto 
in  a  devious,  spinning  movement. 

Although  the  fact  is  not  so  readily  made  visible  to  the 
eye,  the  same  principle  is  illustrated  in  whirling  storms, 
in  which,  as  we  shall  hereafter  note  with  more  detail,  the 
air  next  the  surface  of  the  earth  is  moving  in  toward  a 
kind  of  chimney  by  which  it  escapes  to  the  upper  regions 
of  the  atmosphere.  A  study  of  cyclones  and  tornadoes, 
or  even  of  the  little  air-whirls  which  in  hot  weather  lift 
the  dust  of  our  streets,  shows  that  the  particles  of  the 
atmosphere  in  rushing  in  toward  the  centre  of  upward 
movement  take  on  the  same  whirling  motion  as  do  the 
molecules  of  water  in  the  basin — in  fact,  the  two  actions 
are  perfectly  comparable  in  all  essential  regards,  except 
that  the  fluid  is  moving  downward,  while  the  air  flows 
upward.  Briefly  stated,  the  reason  for  the  movement  of 
fluid  and  gas  in  the  whirling  way  is  as  follows:  If  every 
particle  on  its  way  to  the  centre  moved  on  a  perfectly 
straight  line  toward  the  point  of  escape,  the  flow  would  be 
directly  converging,  and  the  paths  followed  would  resem- 
ble the  spokes  of  a  wheel.  But  when  by  chance  one  of 
the  particles  sways  ever  so  little  to  one  side  of  the  direct 
way,  a  slight  lateral  motion  would  necessarily  be  estab- 
lished. This  movement  would  be  due  to  the  fact  that  the 
particle  which  pursued  the  curved  line  would  press  against 
the  particles  on  the  out-curved  side  of  its  path — or,  in  other 
Avords,  shove  them  a  little  in  that  direction — to  the  extent 
that  they  departed  from  the  direct  line  they  would  in  turn 
communicate  the  shoving  to  the  next  beyond.  When  two 
particles  are  thus  shoving  on  one  side  of  their  paths,  the 
action  which  makes  for  revolution  is  doubled,  and,  as  we 
readily  see,  the  whole  mass  may  in  this  way  become  quickly 
affected,  the  particles  driven  out  of  their  path,  moving  in  a 
curve  toward  the  centre.  We  also  see  that  the  action  is 
accumulative:  the  more  curved  the  path  of  each  particle, the 
more  effectively  it  shoves;  and  so,  in  the  case  of  the  basin, 
we  see  the  whirling  rapidly  developed  before  our  eyes. 


THE  STELLAR  REALM.  37 

In  falling  in  toward  the  centre  the  particles  of  star 
dust  or  vapour  would  no  more  have  been  able  one  and  all 
to  pursue  a  perfectly  straight  line  than  the  particles  of 
water  in  the  basin.  If  a  man  should  spend  his  lifetime  in 
filling  and  emptying  such  a  vessel,  it  is  safe  to  say  that  he 
would  never  fail  to  observe  the  whirling  movement.  As 
the  particles  of  matter  in  the  nebular  mass  which  was  to 
become  a  solar  system  are  inconceivably  greater  than  those 
of  water  in  the  basin,  or  those  of  air  in  the  atmospheric 
whirl,  the  chance  of  the  whirling  taking  place  in  the  heav- 
enly bodies  is  so  great  that  we  may  assume  that  it  would 
inevitably  occur. 

As  the  vapours  in  the  olden  day  tended  in  toward  the 
centre  of  our  solar  system,  and  the  mass  revolved,  there  is 
reason  to  believe  that  ringlike  separations  took  place  in  it. 
Whirling  in  the  manner  indicated,  the  mass  of  vapour  or 
dust  would  flatten  into  a  disk  or  a  body  of  circular  shape, 
with  much  the  greater  diameter  in  the  plane  of  its  whirling. 
As  the  process  of  concentration  went  on,  this  disk  is  sup- 
posed to  have  divided  into  ringlike  masses,  some  approach 
to  which  we  can  discern  in  the  existing  nebulae,  which  here 
and  there  among  the  farther  fixed  stars  appear  to  be  un- 
dergoing such  stages  of  development  toward  solar  systems. 
It  is  reasonably  supposed  that  after  these  rings  had  been 
developed  they  would  break  to  pieces,  the  matter  in  them 
gathering  into  a  sphere,  which  in  time  was  to  become  a 
planet.  The  outermost  of  these  rings  led  to  the  forma- 
tion of  the  planet  farthest  from  the  sun,  and  was  probably 
the  first  to  separate  from  the  parent  mass.  Then  in  suc- 
cession rings  were  formed  inwardly,  each  leading  in  turn 
to  the  creation  of  another  planet,  the  sun  itself  being 
the  remnant,  by  far  the  greater  part  of  the  whole  mass  of 
matter,  which  did  not  separate  in  the  manner  described, 
but  concentrated  on  its  centre.  Each  of  these  planetary 
aggregations  of  vapour  tended  to  develop,  as  it  whirled 
upon  its  centre,  rings  of  its  own,  which  in  turn  formed, 
by  breaking  and  concentrating,  the  satellites  or  moons 


38  OUTLINES  OF  THE  EARTH'S  HISTORY. 

which  attend  the  earth,  as  they  do  aU  the  planets  which 
lie  farther  away  from  the  sun  than  our  sphere. 


Fig.  1.— Saturn,  Jan.  26,  1889  (Antoniadi). 

As  if  to  prove  that  the  planets  and  moons  of  the  solar 
system  were  formed  somewhat  in  the  manner  in  which 
we  have  described  it,  one  of  these  spheres,  Saturn,  retains 
a  ring,  or  rather  a  band  which  appears  to  be  divided  ob- 
scurely into  several  rings  which  lie  between  its  group  of 
satellites  and  the  main  sphere.  How  this  ring  has  been 
preserved  when  all  the  others  have  disappeared,  and  what 
is  the  exact  constitution  of  the  mass,  is  not  yet  well  ascer- 
tained. It  seems  clear,  however,  that  it  can  not  be  com- 
posed of  solid  matter.  It  is  either  in  the  form  of  dust  or 
of  small  spheres,  which  are  free  to  move  on  each  other; 
otherwise,  as  computation  shows,  the  strains  due  to  the  at- 
traction which  Saturn  itself  and  its  moons  exercise  upon 
it  would  serve  to  break  it  in  pieces.  Although  this  ring 
theory  of  the  formation  of  the  planets  and  satellites  is  not 
completely  proved,  the  occurrence  of  such  a  structure  as 
that  which  girdles  Saturn  affords  presumptive  evidence 
that  it  is  true.  Taken  in  connection  with  what  we  know 
of  the  nebulae,  the  proof  of  Laplace's  nebular  hypothesis 
may  fairly  be  regarded  as  complete. 

It  should  be  said  that  some  of  the  fixed  stars  are  not 
isolated  suns  like  our  own,  but  are  composed  of  two  great 


THE  STELLAR  REALM.  39 

spheres  revolving  about  one  another;  hence  they  are 
termed  double  stars.  The  motions  of  these  bodies  are 
very  peculiar,  and  their  conditions  show  us  that  it  is  not 
well  to  suppose  that  the  solar  system  in  which  we  dwell 
is  the  only  type  of  order  which  prevails  in  the  celestial 
families;  there  may,  indeed,  be  other  variations  as  yet 
undetected.  Still,  these  differences  throw  no  doubt  on 
the  essential  truth  of  the  theory  as  to  the  process  of  de- 
velopment of  the  celestial  systems.  Though  there  is  much 
room  for  debate  as  to  the  details  of  the  work  there,  the  gen- 
eral truth  of  the  theory  is  accepted  by  nearly  all  the  stu- 
dents of  the  problem. 

A  peculiar  advantage  of  the  nebular  hypothesis  is  that 
it  serves  to  account  for  the  energy  which  appears  as  light 
and  heat  in  the  sun  and  the  fixed  stars,  as  well  as  that 
which  still  abides  in  the  mass  of  our  earth,  and  doubtless 
also  in  the  other  large  planets.  When  the  matter  of  which 
these  spheres  were  composed  was  disseminated  through 
the  realms  of  space,  it  is  supposed  to  have  had  no  positive 
temperature,  and  to  have  been  dark,  realizing  the  concep- 
tion which  appears  in  the  first  chapter  of  Genesis,  "  with- 
out form,  and  void."  With  each  stage  of  the  falling  in 
toward  the  solar  centres  what  is  called  the  "  energy  of  posi- 
tion "  of  this  original  matter  became  converted  into  light 
and  heat.  To  understand  how  this  took  place,  the  reader 
should  consider  certain  simple  yet  noble  generalizations 
of  physics.  We  readily  recognise  the  fact  that  when  a 
hammer  falls  often  on  an  anvil  it  heats  itself  and  the  metal 
on  which  it  strikes.  Those  who  have  been  able  to  observe 
the  descent  of  meteoric  stones  from  the  heavens  have  re- 
marked that  when  they  came  to  the  earth  they  were,  on 
their  surfaces  at  least,  exceedingly  hot.  Any  one  may  ob- 
serve shining  meteors  now  and  then  flashing  in  the  sky. 
These  are  known  commonly  to  be  very  small  bits  of  mat- 
ter, probably  not  larger  than  grains  of  sand,  which,  rush- 
ing into  our  atmosphere,  are  so  heated  by  the  friction 
which  they  encounter  that  they  burn  to  a  gas  or  vapour 
4 


40  OUTLINES  OF  THE  EARTH'S  HISTORY. 

before  they  attain  the  earth.  As  we  know  that  these  par- 
ticles come  from  the  starry  spaces,  where  the  temperature 
is  somewhere  near  500°  below  0°  Fahr.,  it  is  evident  that 
the  light  and  heat  are  not  brought  with  them  into  the  at- 
mosphere; it  can  only  be  explained  by  the  fact  that  when 
they  enter  the  air  they  are  moving  at  an  average  speed 
of  about  twenty  miles  a  second,  and  that  the  energy  which 
this  motion  represents  is  by  the  resistance  which  the  body 
encounters  converted  into  heat.  This  fact  will  help  us  to 
understand  how,  as  the  original  star  dust  fell  in  toward  the 
centre  of  attraction,  it  was  able  to  convert  what  we  have 
termed  the  energy  of  position  into  temperature.  We  see 
clearly  that  every  such  particle  of  dust  or  larger  bit  of 
matter  which  falls  upon  the  earth  brings  about  the  develop- 
ment of  heat,  even  though  it  does  not  actually  strike  upon 
the  solid  mass  of  our  sphere.  The  conception  of  what  took 
place  in  the  consolidation  of  the  originally  disseminated 
materials  of  the  sun  and  planets  can  be  somewhat  helped 
by  a  simple  experiment.  If  we  fit  a  piston  closely  into  a 
cylinder,  and  then  suddenly  drive  it  down  with  a  heavy 
blow,  the  compressed  air  is  so  heated  that  it  may  be  made 
to  communicate  fire.  If  the  piston  should  be  slowly  moved, 
the  same  amount  of  heat  would  be  generated,  or,  as  we  may 
better  say,  liberated  by  the  compression,  though  the  effect 
would  not  be  so  striking.  A  host  of  experiments  show 
that  when  a  given  mass  of  matter  is  brought  to  occupy 
a  less  space  the  effect  is  in  practically  all  cases  to  increase 
the  temperature.  The  energy  which  kept  the  particles 
apart  is,  when  they  are  driven  together,  converted  into 
heat.  These  two  classes  of  actions  are  somewhat  different 
in  their  nature;  in  the  case  of  the  meteors,  or  the  equiva- 
lent star  dust,  the  coming  together  of  the  particles  is  due 
to  gravitation.  In  the  experiment  with  the  cylinder  above 
described,  the  compression  is  due  to  mechanical  energy, 
a  force  of  another  nature. 

There  is  reason  for  believing  that  all  our  planets,  as 
well  as  the  sun  itself,  and  also  the  myriad  other  orbs  ot 


THE  STELLAR  REALM.  41 

space,  have  all  passed  through  the  stages  of  a  transition 
in  which  a  continually  concentrating  vapour,  drawn  to- 
gether by  gravitation,  became  progressively  hotter  and 
more  dense  until  it  assumed  the  condition  of  a  fluid.  This 
fluid  gradually  parted  with  its  heat  to  the  cold  spaces  of 
the  heavens,  and  became  more  and  more  concentrated  and 
of  a  lower  temperature  until  in  the  end,  as  in  the  case  of 
our  earth  and  of  other  planets,  it  ceased  to  glow  on  the 
outside,  though  it  remained  intensely  heated  in  the  inner 
parts.  It  is  easy  to  see  that  the  rate  of  this  cooling  would 
be  in  some  proportion  to  the  size  of  the  sphere.  Thus  the 
earth,  which  is  relatively  small,  has  become  relatively  cold, 
while  the  sun  itself,  because  of  its  vastly  greater  mass,  still 
retains  an  exceedingly  high  temperature.  The  reason  for 
this  can  readily  be  conceived  by  making  a  comparison  of 
the  rate  of  cooling  which  occurs  in  many  of  our  ordinary 
experiences.  Thus  a  vial  of  hot  water  will  quickly  come 
down  to  the  temperature  of  the  air,  while  a  large  jug  filled 
with  the  fluid  at  the  same  temperature  will  retain  its  heat 
many  times  as  long.  The  reason  for  this  rests  upon  the 
simple  principle  that  the  contents  of  a  sphere  increase 
with  its  enlargement  more  rapidly  than  the  surface  through 
which  the  cooling  takes  place. 

The  modern  studies  on  the  physical  history  of  the  sun 
and  other  celestial  bodies  show  that  their  original  store 
of  heat  is  constantly  flowing  away  into  the  empty  realms 
of  space.  The  rate  at  which  this  form  of  energy  goes  away 
from  the  sun  is  vast  beyond  the  powers  of  the  imagination 
to  conceive;  thus,  in  the  case  of  our  earth,  which  viewed 
from  the  sun  would  appear  no  more  than  a  small  star,  the 
amount  of  heat  which  falls  upon  it  from  the  great  centre 
is  enough  each  day  to  melt,  if  it  all  could  be  put  to  such 
work,  about  eight  thousand  cubic  miles  of  ice.  Yet  the 
earth  receives  only  g, 170,000,000  P^^t  of  the  solar  radia- 
tion. The  greater  part  of  this  solar  heat — in  fact,  we  may 
say  nearly  all  of  it — slips  by  the  few  and  relatively  small 
planets  and  disappears  in  the  great  void. 


42  OUTLINES  OF  THE  EARTH'S  HISTORY. 

The  destiny  of  all  the  celestial  spheres  seems  in  time 
to  be  that  they  shall  become  cooled  down  to  a  temperature 
far  below  anything  which  is  now  experienced  on  this  earth. 
Even  the  sun,  though  its  heat  will  doubtless  endure  for 
millions  of  years  to  come,  must  in  time,  so  far  as  we  can 
see,  become  dark  and  cold.  So  far  as  we  know,  we  can 
perceive  no  certain  method  by  which  the  life  of  the  slowly 
decaying  suns  can  be  restored.  It  has,  however,  been  sug- 
gested that  in  many  cases  a  planetary  system  which  has 
attained  the  lifeless  and  li'ghtless  stage  may  by  collision 
with  some  other  association  of  spheres  be  by  the  blow  re- 
stored to  its  previous  state  of  vapour,  the  Joint  mass  of  the 
colliding  systems  once  again  to  resume  the  process  of  con- 
centration through  which  it  had  gone  before.  Now  and 
then  stars  have  been  seen  to  flash  suddenly  into  great  bril- 
liancy in  a  way  which  suggests  that  possibly  their  heat  had 
been  refreshed  by  a  collision  with  some  great  mass  which 
had  fallen  into  them  from  the  celestial  spaces.  There  is 
room  for  much  speculation  in  this  field,  but  no  certainty 
appears  to  be  attainable. 

The  ancients  believed  that  light  and  heat  were  emana- 
tions which  were  given  off  from  the  bodies  that  yielded 
them  substantially  as  odours  are  given  forth  by  many  sub- 
stances. Since  the  days  of  Newton  inquiry  has  forced  us 
to  the  conviction  that  these  effects  of  temperature  are  pro- 
duced by  vibrations  having  the  general  character  of  waves, 
which  are  sent  through  the  spaces  with  great  celerity. 
When  a  ray  of  light  departs  from  the  sun  or  other  luminous 
body,  it  does  not  convey  any  part  of  the  mass;  it  transmits 
only  motion.  A  conception  of  the  action  can  perhaps  best 
be  formed  by  suspending  a  number  of  balls  of  ivory,  stone, 
or  other  hard  substance  each  by  a  cord,  the  series  so  ar- 
ranged that  they  touch  each  other.  Then  striking  a  blow 
against  one  end  of  the  line,  we  observe  that  the  ball  at  the 
farther  end  of  the  line  is  set  in  motion,  swinging  a  little 
away  from  the  place  it  occupied  before.  The  movement 
of  the  intermediate  balls  may  be  so  slight  as  to  escape 


THE  STELLAR  REALM.  43 

attention.  We  thus  perceive  that  energy  can  be  trans- 
mitted from  one  to  another  of  these  little  spheres.  Close 
observation  shows  us  that  under  the  impulse  which  the 
blow  gives  each  separate  body  is  made  to  sway  within  itself 
much  in  the  manner  of  a  bell  when  it  is  rung,  and  that  the 
movement  is  transmitted  to  the  object  with  which  it  is 
in  contact.  In  passing  from  the  sun  to  the  earth,  the  light 
and  heat  traverse  a  space  which  we  know  to  be  substantially 
destitute  of  any  such  materials  as  make  up  the  mass  of  the 
earth  or  the  sun.  Judged  by  the  standards  which  we  can 
apply,  this  space  must  be  essentially  empty.  Yet  because 
motions  go  through  it,  we  have  to  believe  that  it  is  occupied 
by  something  which  has  certain  of  the  properties  of  matter. 
It  has,  indeed,  one  of  the  most  important  properties  of 
all  substances,  in  that  it  can  vibrate.  This  practically  un- 
known thing  is  called  ether. 

The  first  important  observational  work  done  by  the 
ancients  led  them  to  perceive  that  there  was  a  very  char- 
acteristic difference  between  the  planets  and  the  fixed  stars. 
They  noted  the  fact  that  the  planets  wandered  in  a  cease- 
less way  across  the  heavens,  while  the  fixed  stars  showed 
little  trace  of  changing  position  in  relation  to  one  another. 
For  a  long  time  it  was  believed  that  these,  as  well  as  the 
remoter  fixed  stars,  revolved  about  the  earth.  This  error, 
though  great,  is  perfectly  comprehensible,  for  the  evident 
appearance  of  the  movement  is  substantially  what  would 
be  brought  about  if  they  really  coursed  around  our  sphere. 
It  was  only  when  the  true  nature  of  the  earth  and  its  rela- 
tions to  the  sun  were  understood  that  men  could  correct 
this  first  view.  It  was  not,  indeed,  until  relatively  modern 
times  that  the  solar  system  came  to  be  perceived  as  some- 
thing independent  and  widely  detached  from  the  fixed 
stars  system;  that  the  spaces  which  separate  the  members 
of  our  own  solar  family,  inconceivably  great  as  they  are, 
are  but  trifling  as  compared  with  the  intervals  which  part 
us  from  the  nearer  fixed  stars.  At  this  stage  of  our  knowl- 
edge men  came  to  the  noble  suggestion  that  each  of  the 


44  OUTLINES  OF  THE  EARTH'S  HISTORY. 

fixed  stars  was  itself  a  sun,  eacli  of  the  myriad  probably 
attended  by  planetary  bodies  such  as  exist  about  our  own 
luminary. 

It  will  be  well  for  the  student  to  take  an  imaginary 
journey  from  the  sun  forth  into  space,  along  the  plane  in 
which  extends  that  vast  aggregation  of  stars  which  we  term 
the  Milky  Way.  Let  him  suppose  that  his  journey  could 
be  made  with  something  like  the  speed  of  light,  or,  say, 
at  the  rate  of  about  two  hundred  thousand  miles  a  second. 
It  ie  fit  that  the  imagination,  which  is  free  to  go  through 
all  things,  should  essay  such  excursions.  On  the  fancied 
outgoing,  the  observer  would  pass  the  interval  between 
the  sun  and  the  earth  in  about  eight  minutes.  It  would 
require  some  hours  before  he  attained  to  the  outer  limit 
of  the  solar  system.  On  his  direct  way  he  would  pass  the 
orbits  of  the  several  planets.  Some  would  have  their 
courses  on  one  side  or  the  other  of  his  path;  we  should  say 
above  or  below,  but  for  the  fact  that  we  leave  these  terms 
behind  in  the  celestial  realm.  On  the  margin  of  the  solar 
system  the  sun  would  appear  shrunken  to  the  state  where 
it  was  hardly  greater  than  the  more  brilliant  of  the  other 
fixed  stars.  The  onward  path  would  then  lead  through 
a  void  which  it  would  require  years  to  traverse.  Gradually 
the  sun  which  happened  to  lie  most  directly  in  his  path 
would  grow  larger;  with  nearer  approach,  it  would  disclose 
its  planets.  Supposing  that  the  way  led  through  this  solar 
system,  there  would  doubtless  be  revealed  planets  and 
satellites  in  their  order  somewhat  resembling  those  of  our 
own  solar  family,  yet  there  would  doubtless  be  many  sur- 
prises in  the  view.  Arriving  near  the  first  sun  to  be  visited, 
though  the  heavens  would  have  changed  their  shape,  all 
the  existing  constellations  having  altered  with  the  change 
in  the  point  of  view,  there  would  still  be  one  familiar  ele- 
ment in  that  the  new-found  planets  would  be  near  b}^,  and 
the  nearest  fixed  stars  far  away  in  the  firmament. 

With  the  speed  of  light  a  stellar  voyage  could  be  taken 
along  the  path  of  the  Milky  Way,  which  would  endure  for 


THE  STELLAR  REALM.  45 

thousands  of  years.  Through  all  the  course  the  journeyer 
would  perceive  the  same  vast  girdle  of  stars,  faint  because 
they  were  far  away,  which  gives  the  dim  light  of  our 
galaxy.  At  no  point  is  it  probable  that  he  would  find 
the  separate  suns  much  more  aggregated  or  greatly  farther 
apart  than  they  are  in  that  part  of  the  Milky  Way  which 
our  sun  now  occupies.  Looking  forth  on  either  side  of 
the  "  galactic  plane,"  there  would  be  the  same  scattering 
of  stars  which  we  now  behold  when  we  gaze  at  right  angles 
to  the  way  we  are  supposing  the  spirit  to  traverse. 

As  the  form  of  the  Milky  Way  is  irregular,  the  mass, 
indeed,  having  certain  curious  divisions  and  branches,  it 
well  might  be  that  the  supposed  path  would  occasionally 
pass  on  one  or  the  other  side  of  the  vast  star  layer.  In 
such  positions  the  eye  would  look  forth  into  an  empty 
firmament,  except  that  there  might  be  in  the  far  away, 
tens  of  thousands  of  years  perhaps  at  the  rate  that  light 
travels  away  from  the  observer,  other  galaxies  or  Milky 
Ways  essentially  like  that  which  he  was  traversing.  At 
some  point  the  journeyer  would  attain  the  margin  of  our 
star  stratum,  whence  again  he  would  look  forth  into  the 
unpeopled  heavens,  though  even  there  he  might  discern 
other  remote  star  groups  separated  from  his  own  by  great 
void  intervals. 

The  revelations  of  the  telescope  show  us  certain  fea- 
tures in  the  constitution  and  movements  of  the  fixed  stars 
which  now  demand  our  attention.  In  the  first  place,  it  is 
plain  that  not  all  of  these  bodies  are  in  the  same  physical 
condition.  Though  the  greater  part  of  these  distant  lumi- 
nous masses  are  evidently  in  the  state  of  aggregation  dis- 
played by  our  own  sun,  many  of  them  retain  more  or  less 
of  that  vaporous,  it  may  be  dustlike,  character  which  we 
suppose  to  have  been  the  ancient  state  of  all  the  matter 
in  the  universe.  Some  of  these  masses  appear  as  faint, 
almost  indistinguishable  clouds,  which  even  to  the  greatest 
telescope  and  the  best-trained  vision  show  no  distinct  fca- 


46  OUTLINES  OF  THE  EARTH'S  HISTORY. 

tures  of  structure.  In  other  cases  the  nebulous  appearance 
is  hardly  more  than  a  mist  about  a  tolerably  distinct  cen- 
tral star.  Yet  again,  and  most  beautifully  in  the  great 
nebula  of  the  constellation  of  Orion,  the  cloudy  mass, 
though  hardly  visible  to  the  naked  eye,  shows  a  division 
into  many  separate  parts,  the  whole  appearing  as  if  in 
process  of  concentration  about  many  distinct  centres. 

The  nebulae  are  reasonably  believed  by  many  astrono- 
mers to  be  examples  of  the  ancient  condition  of  the  phys- 
ical universe,  masses  of  matter  which  for  some  reason  as 
yet  unknown  have  not  progressed  in  their  consolidation  to 
the  point  where  they  have  taken  on  the  characteristics 
of  suns  and  their  attendant  planets. 

Many  of  the  fixed  stars,  the  incomplete  list  of  which 
now  amounts  to  several  hundred,  are  curiously  variable  in 
the  amount  of  light  which  they  send  out  to  the  earth. 
Sometimes  these  variations  are  apparently  irregular,  but 
in  the  greater  number  of  cases  they  have  fixed  periods, 
the  star  waxing  and  waning  at  intervals  varying  from  a  few 
months  to  a  few  years.  Although  some  of  the  sudden 
flashings  forth  of  stars  from  apparent  small  size  to  near 
the  greatest  brilliancy  may  be  due  to  catastrophes  such 
as  might  be  brought  about  by  the  sudden  falling  in  of 
masses  of  matter  upon  the  luminous  spheres,  it  is  more 
likely  that  the  changes  which  we  observe  are  due  to  the 
fact  that  two  suns  revolving  around  a  common  centre  are 
in  different  stages  of  extinction.  It  may  well  be  that 
one  of  these  orbs,  presumably  the  smaller,  has  so  far  lost 
temperature  that  it  has  ceased  to  glow.  If  in  its  revolu- 
tion it  regularly  comes  between  the  earth  and  its  luminous 
companion,  the  effect  would  be  to  give  about  such  a  change 
in  the  amount  of  light  as  we  observe. 

The  supposition  that  a  bright  sun  and  a  relatively  dark 
sun  might  revolve  around  a  common  centre  of  gravity  may 
at  first  sight  seem  improbable.  The  fact  is,  however,  that 
imperfect  as  our  observations  on  the  stars  really  are,  we 
know  many  instances  in  which  this  kind  of  revolution  of 


THE  STELLAR  REALM.  47 

one  star  about  another  takes  place.  In  some  cases  these 
stars  are  of  the  same  brilliancy,  but  in  others  one  of  the 
lights  is  much  brighter  than  the  other.  From  this  condi- 
tion to  the  state  where  one  of  the  stars  is  so  nearly  dark 
as  to  be  invisible,  the  transition  is  but  slight.  In  a  word, 
the  evidence  goes  to  show  that  while  we  see  only  the  lumi- 
nous orbs  of  space,  the  dark  bodies  which  people  the  heav- 
ens are  perhaps  as  numerous  as  those  which  send  us  light, 
and  therefore  appear  as  stars. 

Besides  the  greater  spheres  of  space,  there  is  a  vast  host 
of  lesser  bodies,  the  meteorites  and  comets,  which  appear 
to  be  in  part  members  of  our  solar  system,  and  perhaps  of 
other  similar  systems,  and  in  part  wanderers  in  the  vast 
realm  which  intervenes  between  the  solar  systems.  Of 
these  we  will  first  consider  the  meteors,  of  which  we  know 
by  far  the  most;  though  even  of  them,  as  we  shall  see,  our 
knowledge  is  limited. 

From  time  to  time  on  any  starry  night,  and  particularly 
in  certain  periods  of  the  year,  we  may  behold,  at  the  dis- 
tance of  fifty  or  more  miles  above  the  surface  of  the  earth, 
what  are  commonly  called  "  shooting  stars."  The  most  of 
these  flashing  meteors  are  evidently  very  small,  probably 
not  larger  than  tiny  sand  grains,  possibly  no  greater  than 
the  fragments  which  would  be  termed  dust.  They  enter 
the  air  at  a  speed  of  about  thirty  miles  a  second.  They 
are  so  small  that  they  burn  to  vapour  in  the  very  great  heat 
arising  from  their  friction  on  the  air,  and  do  not  attain 
the  surface  of  the  earth.  These  are  so  numerous  that,  on 
the  average,  some  hundreds  of  thousands  probably  strike 
the  earth's  atmosphere  each  day.  From  time  to  time  larger 
badies  fall — bodies  which  are  of  sufficient  bulk  not  to  be 
burned  up  in  the  air,  but  which  descend  to  the  ground. 
These  may  be  from  the  smallest  size  which  may  be  ob- 
served to  masses  of  many  hundred  pounds  in  weight.  These 
are  far  less  numerous  than  the  dust  meteorites;  it  is  prob- 
able, however,  that  several  hundred  fragments  each  year 
attain  the  earth's  surface.    They  come  from  various  direc- 


48  OUTLINES  OF  THE  EARTH'S  HISTORY. 

tions  of  space,  and  there  is  as  yet  no  means  of  determining 
whether  they  were  formed  in  some  manner  within  our 
planetary  system  or  whether  they  wander  to  us  from  re- 
moter realms.  We  know  that  they  are  in  part  composed  of 
metallic  iron  commingled  with  nickel  and  carbon  (some- 
times as  very  small  diamonds)  in  a  way  rarely  if  ever  found 
on  the  surface  of  our  sphere,  and  having  a  structure  sub- 
stantially unknown  in  its  deposits.  In  part  they  are  com- 
posed of  materials  which  somewhat  resemble  certain  lavas. 
It  is  possible  that  these  fragments  of  iron  and  stone  which 
constitute  the  meteorites  have  been  thrown  into  the  plan- 
etary spaces  by  the  volcanic  eruption  of  our  own  and  other 
planets.  If  hurled  forth  with  a  sufficient  energy,  the 
fragments  would  escape  from  the  control  of  the  attrac- 
tion of  the  sphere  whence  they  came,  and  would  become 
independent  wanderers  in  space,  moving  around  the  sun 
in  varied  orbits  until  they  were  again  drawn  in  by  some 
of  the  greater  planets. 

As  they  come  to  us  these  meteorites  often  break  up 
in  the  atmosphere,  the  bits  being  scattered  sometimes  over 
a  wide  area  of  country.  Thus,  in  the  case  of  the  Cocke 
County  meteorite  of  Tennessee,  one  of  the  iron  species, 
the  fragments,  perhaps  thousands  in  number,  which  came 
from  the  explosion  of  the  body  were  scattered  over  an  area 
of  some  thousand  square  miles.  When  they  reach  the  sur- 
face in  their  natural  form,  these  meteors  always  have  a 
curious  wasted  and  indented  appearance,  which  makes  it 
seem  likely  that  they  have  been  subject  to  frequent  col- 
lisions in  their  journeys  after  they  were  formed  by  some 
violent  rending  action. 

In  some  apparent  kinship  with  the  meteorites  may  be 
classed  the  comets.  The  peculiarity  of  these  bodies  is  that 
they  appear  in  most  cases  to  be  more  or  less  completely 
vaporous.  Hushing  down  from  the  depths  of  the  heavens, 
these  bodies  commonly  appear  as  faintly  shining,  cloudlike 
masses.  As  they  move  in  toward  the  sun  long  trails  of 
vapour  stream  back  from  the  somewhat  consolidated  head. 


THE  STELLAR  REALM. 


49 


Swinging  around  that  centre,  they  journey  again  into  the 
outer  realm.    As  they  retreat,  their  tail-like  streamers  ap- 


mmBmmmmaimmmm 

Fig.  2. — The  Great  Comet  of  1811,  one  of  the  many  varied  forms 
of  these  bodies. 


pear  to  gather  again  upon  their  centres,  and  when  they 
fade  from  view  they  are  again  consolidated.    In  some  cases 


50  OUTLINES  OF  THE  EARTH'S  HISTOEY. 

it  has  been  suspected  that  a  part  at  least  of  the  cometary 
mass  was  solid.  The  evidence  goes  to  show,  however,  that 
the  matter  is  in  a  dustlike  or  vaporous  condition,  and  that 
the  weight  of  these  bodies  is  relatively  very  small. 

Owing  to  their  strange  appearance,  comets  were  to  the 
ancients  omens  of  calamity.  Sometimes  they  were  con- 
ceived as  flaming  swords;  their  forms,  indeed,  lend  them- 
selves to  this  imagining.  They  were  thought  to  presage 
war,  famine,  and  the  death  of  kings.  Again,  in  more  mod- 
ern times,  when  they  were  not  regarded  as  portents  of 
calamity,  it  was  feared  that  these  wanderers  moving  vagari- 
ously  through  our  solar  system  might  by  chance  come  in 
contact  with  the  earth  with  disastrous  results.  Such  col- 
lisions are  not  impossible,  for  the  reason  that  the  planets 
would  tend  to  draw  these  errant  bodies  toward  them  if 
they  came  near  their  spheres;  yet  the  chance  of  such 
collisions  happening  to  the  earth  is  so  small  that  they  may 
be  disregarded. 

Motions  of  the  Spheres. 

Although  little  is  known  of  the  motions  which  occur 
among  the  celestial  bodies  beyond  the  sphere  of  our  solar 
family,  that  which  has  been  ascertained  is  of  great  impor- 
tance, and  serves  to  make  it  likely  that  all  the  suns  in  space 
are  upon  swift  journeys  which  in  their  speed  equal,  if  they 
do  not  exceed,  the  rate  of  motion  among  the  planetary 
spheres,  which  may,  in  general,  be  reckoned  at  about 
twenty  miles  a  second.  Our  whole  solar  system  is  journey- 
ing away  from  certain  stars,  and  in  the  direction  of  others 
which  are  situated  in  the  opposite  part  of  the  heavens. 
The  proof  of  this  fact  is  found  in  the  observations  which 
show  that  on  one  side  of  us  the  stars  are  apparently  com- 
ing closer  together,  while  on  the  other  side  they  are  going 
farther  apart.  The  phenomenon,  in  a  word,  is  one  of  per- 
spective, and  may  be  made  real  to  the  understanding  by 
noting  what  takes  place  when  we  travel  down  a  street 


THE  STELLAR  REALM.  51 

along  which  there  are  lights.  We  readily  note  that  these 
lights  appear  to  close  in  behind  us,  and  widen  their  inter- 
vals in  the  direction  in  which  we  journey.  By  such  evi- 
dence astronomers  have  become  convinced  that  our  sphere, 
along .  with  the  sun  which  controls  it,  is  each  second  a 
score  of  miles  away  from  the  point  where  it  was  before. 

There  is  yet  other  and  most  curious  evidence  which 
serves  to  show  that  certain  of  the  stars  are  journeying 
toward  our  part  of  the  heavens  at  great  speed,  while  otheis 
are  moving  away  from  us  by  their  own  proper  motion. 
These  indications  are  derived  from  the  study  of  the  lines  in 
the  light  which  the  spectrum  reveals  to  us  when  critically 
examined.  The  position  of  these  cross  lines  is,  as  we  know, 
affected  by  the  motion  of  the  body  whence  the  light  comes, 
and  by  close  analysis  of  the  facts  it  has  been  pretty  well 
determined  that  the  distortion  in  their  positions  is  due  to 
very  swift  motions  of  the  several  stars.  It  is  not  yet  cer- 
tain whether  these  movements  of  our  sun  and  of  other 
solar  bodies  are  in  straight  lines  or  in  great  circles. 

It  should  be  noted  that,  although  the  evidence  from  the 
spectroscope  serves  to  show  that  the  matter  in  the  stars  is 
akin  to  that  of  our  own  earth,  there  is  reason  to  believe 
that  those  great  spheres  differ  much  from  each  other  in 
magnitude. 

We  have  now  set  forth  some  of  the  important  facts 
exhibited  by  the  stellar  universe.  The  body  of  details  con- 
cerning that  realm  is  vast,  and  the  conclusions  drawn  from 
it  important;  only  a  part,  however,  of  the  matter  with 
which  it  deals  is  of  a  nature  to  be  apprehended  by  the 
student  who  does  not  approach  it  in  a  somewhat  profes- 
sional way.  We  shall  therefore  now  turn  to  a  description 
of  the  portion  of  the  starry  world  which  is  found  in  the 
limits  of  our  solar  system.  There  the  influences  of  the 
several  spheres  upon  our  planet  are  matters  of  vital  im- 
portance; they  in  a  way  affect,  if  they  do  not  control,  all 
the  operations  which  go  on  upon  the  surface  of  the  earth. 


52  OUTLINES  OF  THE  EARTH'S  HISTORY. 


The  Solak  System. 

We  have  seen  that  the  matter  in  the  visible  universe 
everywhere  tends  to  gather  into  vast  associations  which 
appear  to  us  as  stars,  and  that  these  orbs  are  engaged  in 
ceaseless  motion  in  Journeys  through  space.  In  only  one 
of  these  aggregations — that  which  makes  our  own  solar 
system — are  the  bodies  sufficiently  near  to  our  eyes  for  us, 
even  with  the  resources  of  our  telescopes  and  other  instru- 
ments, to  divine  something  of  the  details  which  they  ex- 
hibit. In  studying  what  we  may  concerning  the  family 
of  the  sun,  the  planets,  and  their  satellites,  we  may  reason- 
ably be  assured  that  we  are  tracing  a  history  which  with 
many  differences  is  in  general  repeated  in  the  development 
of  each  star  in  the  firmament.  Therefore  the  inquiry 
is  one  of  vast  range  and  import. 

Following,  as  we  may  reasonably  do,  the  nebular  hy- 
pothesis— a  view  which,  though  not  wholly  proved,  is  emi- 
nently probable — we  may  regard  our  solar  system  as  having 
begun  when  the  matter  of  which  it  is  composed,  then  in 
a  finely  divided,  cloudy  state,  w^as  separated  from  the  simi- 
lar material  which  went  to  make  the  neighbouring  fixed 
stars.  The  period  when  our  solar  system  began  its  indi- 
vidual life  was  remote  beyond  the  possibility  of  conception. 
Naturalists  are  pretty  well  agreed  that  living  beings  began 
to  exist  upon  the  earth  at  least  a  hundred  million  years 
ago;  but  the  beginnings  of  our  solar  system  must  be  placed 
at  a  date  very  many  times  as  remote  from  the  present  day.* 

According  to  the  nebular  theory,  the  original  vapour 
of  the  solar  system  began  to  fall  in  toward  its  centre  and 
to  whirl  about  that  point  at  a  time  long  before  the  mass 

*  Some  astronomers,  particularly  the  distinguished  Professor  New- 
comb,  hold  that  the  sun  can  not  have  been  supplying  heat  as  at 
present  for  more  than  about  ten  million  years,  and  that  all  geological 
time  must  be  thus  limited,  The  geologist  believes  that  this  reckon- 
ing is  fjiir  top  short. 


THE  STELLAR  REALM.  53 

had  shrunk  to  the  present  limits  of  the  solar  sj'stem  as 
defined  by  the  path  of  the  outermost  planets.  At  succes- 
sive stages  of  the  concentration,  rings  after  the  manner 
of  those  of  Saturn  separated  from  the  disklike  mass,  each 
breaking  up  and  consolidating  into  a  body  of  nebulous 
matter  which  followed  in  the  same  path,  generally  forming 
rings  which  became  by  the  same  process  the  moons  or  satel- 
lites of  the  sphere.  In  this  way  the  sun  produced  eight 
planets  which  are  known,  and  possibly  others  of  small  size 
on  the  outer  verge  of  the  system  which  have  eluded  dis- 
covery. According  to  this  view,  the  planetary  masses  were 
born  in  succession,  the  farthest  away  being  the  oldest. 
It  is,  however,  held  by  an  able  authority  that  the  mass  of 
the  solar  system  would  first  form  a  rather  flat  disk,  the 
several  rings  forming  and  breaking  into  planets  at  about 
the  same  time.  The  conditions  in  Saturn,  where  the  inner 
ring  remains  parted,  favours  the  view  just  stated. 

Before  making  a  brief  statement  of  the  several  planets, 
the  asteroids,  and  the  satellites,  it  will  be  well  to  consider 
in  a  general  way  the  motions  of  these  bodies  about  their 
centres  and  about  the  sun.  The  most  characteristic  and 
invariable  of  these  movements  is  that  by  which  each  of  the 
planetary  spheres,  as  well  as  the  satellites,  describes  an 
orbit  around  the  gravitative  centre  which  has  the  most 
influence  upon  it — the  sun.  To  conceive  the  nature  of  this 
movement,  it  will  be  well  to  imagine  a  single  planet  re- 
volving around  the  sun,  each  of  these  bodies  being  perfect 
spheres,  and  the  two  the  only  members  of  the  solar  system. 
In  this  condition  the  attraction  of  the  two  bodies  would 
cause  them  to  circle  around  a  common  centre  of  gravity, 
which,  if  the  planet  were  not  larger  or  the  sun  smaller 
than  is  the  case  in  our  solar  system,  would  lie  within  the 
mass  of  the  sun.  In  proportion  as  the  two  bodies  might 
approach  each  other  in  size,  the  centre  of  gravity  would 
come  the  nearer  to  the  middle  point  in  a  line  connecting 
the  two  spheres.  In  this  condition  of  a  sun  with  a  single 
planet,  whatever  were  the  relative  size  of  sun  and  planet, 


54  OUTLINES  OF  THE  EARTH'S  HISTORY. 

the  orbits  which  they  traverse  would  be  circular.  In  this 
state  of  affairs  it  should  be  noted  that  each  of  the  two 
bodies  would  have  its  plane  of  rotation  permanently  in  the 
same  position.  Even  if  the  spheres  were  more  or  less  flat- 
tened about  the  poles  of  their  axes,  as  is  the  case  with  all 
the  planets  which  we  have  been  able  carefully  to  measure, 
as  well  as  with  the  sun,  provided  the  axes  of  rotation  were 
•precisely  parallel  to  each  other,  the  mutual  attraction  of 
the  masses  would  cause  no  disturbance  of  the  spheres. 
The  same  would  be  the  case  if  the  polar  axis  of  one  sphere 
stood  precisely  at  right  angles  to  that  of  the  other.  If, 
however,  the  spheres  were  somewhat  flattened  at  the  poles, 
and  the  axes  inclined  to  each  other,  then  the  pull  of  one 
mass  on  the  other  would  cause  the  polar  axes  to  keep  up 
a  constant  movement  which  is  called  nutation,  or  nodding. 

The  reason  why  this  nodding  movement  of  the  polar 
axes  would  occur  when  these  lines  were  inclined  to  each 
other  is  not  difficult  to  see  if  we  remember  that  the  attrac- 
tion of  masses  upon  each  other  is  inversely  as  the  square 
of  the  distance;  each  sphere,  pulling  on  the  equatorial 
bulging  of  the  other,  pulls  most  effectively  on  the  part  of 
it  which  is  nearest,  and  tends  to  draw  it  down  toward  its 
centre.  The  result  is  that  the  axes  of  the  attracted  spheres 
are  given  a  wobbling  movement,  such  as  we  may  note  in  the 
spinning  top,  though  in  the  toy  the  cause  of  the  motion  is 
not  that  which  we  are  considering. 

If,  now,  in  that  excellent  field  for  the  experiment  we 
are  essaying,  the  mind's  eye,  we  add  a  second  planet  out- 
side of  the  single  sphere  which  we  have  so  far  supposed  to 
journey  about  the  sun,  or  rather  about  the  common  centre 
of  gravity,  w^e  perceive  at  once  that  we  have  introduced 
an  element  which  leads  to  a  complication  of  much  impor- 
tance. The  new  sphere  would,  of  course,  pull  upon  the 
others  in  the  measure  of  its  gravitative  value — i.  e.,  its 
weight.  The  centre  of  gravity  of  the  system  would  now 
be  determined  not  by  two  distinct  bodies,  but  by  three. 
If  we  conceive  the  second  planet  to  journey  around  the 


THE  STELLAR  REALM.  55 

sun  at  such  a  rate  that  a  straiglit  line  always  connected 
the  centres  of  the  three  orbs,  then  the  only  effect  on  their 
gravitative  centre  would  be  to  draw  the  first-mentioned 
planet  a  little  farther  away  from  the  centre  of  the  sun; 
but  in  our  own  solar  system,  and  probably  in  all 
others,  this  supposition  is  inadmissible,  because  the  plan- 
ets have  longer  journeys  to  go  and  also  move  slower, 
the  farther  they  are  from  the  sun.  Thus  Mercury 
completes  the  circle  of  its  year  in  eighty-eight  of  our 
days,  while  the  outermost  planet  requires  sixty  thou- 
sand days  (more  than  one  hundred  and  sixty-four  years) 
for  the  same  task.  The  result  is  not  only  that  the  centre 
of  gravity  of  the  system  is  somewhat  displaced — itself  a 
matter  of  no  great  account — but  also  that  the  orbit  of  the 
original  planet  ceases  to  be  circled  and  becomes  elliptical, 
and  this  for  the  evident  reason  that  the  sphere  will  be  drawn 
somewhat  away  from  the  sun  when  the  second  planet  hap- 
pens to  lie  in  the  part  of  its  orbit  immediately  outside  of  its 
position,  in  which  case  the  pull  is  away  from  the  solar 
centre;  while,  on  the  other  hand,  when  the  new  planet 
was  on  the  other  side  of  the  sun,  its  pull  would  serve  to 
intensify  the  attraction  which  drew  the  first  sphere  toward 
the  centre  of  gravity.  As  the  pulling  action  of  the  three 
bodies  upon  each  other,  as  well  as  upon  their  equatorial 
protuberances,  would  vary  with  every  change  in  their  rela- 
tive position,  however  slight,  the  variations  in  the  form 
of  their  orbits,  even  if  the  spheres  were  but  three  in  num- 
ber, would  be  very  important.  The  consequences  of  these 
perturbations  will  appear  in  the  sequel. 

In  our  solar  system,  though  there  are  but  eight  great 
planets,  the  group  of  asteroids,  and  perhaps  a  score  of 
satellites,  the  variety  of  orbital  and  axial  movement  which 
is  developed  taxes  the  computing  genius  of  the  ablest 
astronomer.  The  path  which  our  earth  follows  around 
the  sun,  though  it  may  in  general  and  for  convenience 
be  described  as  a  variable  ellipse,  is,  in  fact,  a  line  of 
such  complication  that  if  we  should  essay  a  diagram  of  it 
5 


56  OUTLINES  OF  THE  EARTH^S  HISTORY. 

on  the  scale  of  this  page  it  would  not  be  possible  to  repre- 
sent any  considerable  part  of  its  deviations.  These,  in 
fact,  would  elude  depiction,  even  if  the  draughtsman  had 
a  sheet  for  his  drawing  as  large  as  the  orbit  itself,  for 
every  particle  of  matter  in  space,  even  if  it  be  lodged  be- 
yond the  limits  of  the  farthest  stars  revealed  to  us  by  the 
telescope,  exercises  a  certain  attraction,  which,  however 
small,  is  effective  on  the  mass  of  the  earth.  Science  has 
to  render  its  conclusions  in  general  terms,  and  we  can 
safely  take  them  as  such;  but  in  this,  as  in  other  instances, 
it  is  well  to  qualify  our  acceptance  of  the  statements  by 
the  memory  that  all  things  are  infinitely  more  complicated 
than  we  can  possibly  conceive  or  represent  them  to  be. 

We  have  next  to  consider  the  rotations  of  the  planetary 
spheres  upon  their  axes,  together  with  the  similar  move- 
ment, or  lack  of  it,  in  the  case  of  their  satellites.  This 
rotation,  according  to  the  nebular  hypothesis,  may  be  ex- 
plained by  the  movements  which  would  set  up  in  the  share 
of  matter  which  was  at  first  a  ring  of  the  solar  nebulae, 
and  which  afterward  gathered  into  the  planetary  aggrega- 
tion. The  way  of  it  may  be  briefly  set  forth  as  follows: 
Such  a  ring  doubtless  had  a  diameter  of  some  million  miles; 
we  readily  perceive  that  the  particles  of  matter  in  the  outer 
part  of  the  belt  would  have  a  swifter  movement  around 
the  sun  than  those  on  the  inside.  When  by  some  disturb- 
ance, as  possibly  by  the  passage  of  a  great  meteoric  body 
of  a  considerable  gravitative  power,  this  ring  was  broken 
in  two,  the  particles  composing  it  on  either  side  would, 
because  of  their  mutual  attraction,  tend  to  draw  away  from 
the  breach,  widening  that  gap  until  the  matter  of  the 
broken  ring  was  aggregated  into  a  sphere  of  the  star  dust 
or  vapour.  When  the  nebulous  matter  originally  in  the 
ring  became  aggregated  into  a  spherical  form,  it  would, 
on  account  of  the  different  rates  at  which  the  particles 
were  moving  when  they  came  together,  be  the  surer  to  fall 
in  toward  the  centre,  not  in  straight  lines,  but  in  curves — 
in  other  words,  the  mass  would  necessarily  take  on  a  move- 


THE  STELLAR  REALM.  57 

ment  of  rotation  essentially  like  that  which  we  have  de- 
scribed in  setting  forth  the  nebular  hypothesis. 

In  the  stages  of  concentration  the  planetary  nebulas 
might  well  repeat  those  through' which  the  greater  solar 
mass  proceeded.  If  the  volume  of  the  material  were  great, 
subordinate  rings  would  be  formed,  which  when  they  broke 
and  concentrated  would  constitute  secondary  planets  or 
satellites,  such  as  our  moon.  For  some  reason  as  yet  un- 
known the  outer  planets — in  fact,  all  those  in  the  solar 
system  except  the  two  inner,  Venus  and  Mercury  and  the 
asteroids — formed  such  attendants.  All  these  satellite- 
forming  rings  have  broken  and  concentrated  except  the 
inner  of  Saturn,  which  remains  as  an  intellectual  treasure 
of  the  solar  system  to  show  the  history  of  its  development. 

To  the  student  who  is  not  seeking  the  fulness  of  knowl- 
edge which  astronomy  has  to  offer,  but  desires  only  to  ac- 
quaint himself  with  the  more  critical  and  important  of 
the  heavenly  i)henomena  which  help  to  explain  the  earth, 
these  features  of  planetary  movement  should  prove  espe- 
cially interesting  for  the  reason  that  they  shape  the  history 
of  the  spheres.  As  we  shall  hereafter  see,  the  machinery 
of  the  earth's  surface,  all  the  life  which  it  bears,  its  winds 
and  rains — everything,  indeed,  save  the  actions  which  go  on 
in  the  depths  of  the  sphere— is  determined  by  the  heat  and 
light  which  come  from  the  sun.  The  conditions  under 
which  this  vivifying  tide  is  received  have  their  origin  in 
the  planetary  motion.  If  our  earth's  path  around  the 
centre  of  the  system  was  a  perfect  circle,  and  if  its  polar 
axis  lay  at  right  angles  to  the  plane  of  its  journey,  the 
share  of  light  and  heat  which  would  fall  upon  any  one 
point  on  the  sphere  would  be  perfectly  uniform.  There 
would  be  no  variations  in  the  length  of  day  or  night;  no 
changes  in  the  seasons;  the  winds  everywhere  would  blow 
with  exceeding  steadiness — in  fact,  the  present  atmospheric 
confusion  would  be  reduced  to  something  like  order.  From 
age  to  age,  except  so  far  as  the  sun  itself  might  vary  in 
the  amount  of  energy  which  it  radiated,  or  lands  rose  up 


58 


OUTLINES  OF  THE  EARTH'S  HISTORY. 


into  the  air  or  sunk  down  toward  the  sea  level,  the  climate 
of  each  region  would  be  perfectly  stable.    In  the  existing 

conditions  the  influences 
bring  about  unending  vari- 
ety. First  of  all,  the  in- 
clined position  of  the  polar 
axis  causes  the  sun  appar- 
ently to  move  across  the 
heavens,  so  that  it  comes 
in  an  overhead  position 
once  or  twice  in  the  year 
in  quite  half  the  area  of 
the  lands  and  seas.  This 
apparent  swaying  to  and 
fro  of  the  sun,  due  to  the 
inclination  of  the  axis  of 
rotation,  also  affects  the 
width  of  the  climatal  belts 
on  either  side  of  the  equa- 
tor, so  that  all  parts  of  the 
earth  receive  a  considerable 
share  of  the  sun's  influence. 
If  the  axis  of  the  earth's 
rotation  were  at  right  an- 
gles to  the  plane  of  its 
orbit,  there  would  be  a  nar- 
row belt  of  high  tempera- 
ture about  the  equator, 
north  and  south  of  which 
the  heat  would  grade  off 
until  at  about  the  parallels 
of  fifty  degrees  we  should 
find  a  cold  so  considerable 
and  uniform  that  life  would 
probably  fade  away;  and  from  those  parallels  to  the  poles 
the  conditions  would  be  those  of  permanent  frost,  and  of 
days  which  would  darken  into  the  enduring  night  or  twi- 


THE  STELLAR  REALM.  59 

light  in  the  realm  of  the  far  north  and  south.  Thus  the 
wide  habitability  of  the  earth  is  an  effect  arising  from  the 
inclination  of  its  polar  axis. 

As  the  most  valuable  impression  which  the  student 
can  receive  from  his  study  of  Nature  is  that  sense  of  the 
order  which  has  made  possible  all  life,  including  his  own, 
it  will  be  well  for  him  to  imagine,  as  he  may  readily  do, 
what  would  be  the  effect  arising  from  changes  in  relations 
of  earth  and  sun.  Bringing  the  earth's  axis  in  imagination 
into  a  position  at  right  angles  to  the  plane  of  the  orbit, 
he  will  see  that  the  effect  would  be  to  intensify  the  equa- 
torial heat,  and  to  rob  the  high  latitudes  of  the  share  which 
they  now  have.  On  moving  the  axis  gradually  to  positions 
where  it  approaches  the  plane  of  the  orbit,  he  will  note 
that  each  stage  of  the  change  widens  the  tropic  belt. 
Bringing  the  polar  axis  down  to  the  plane  of  the  orbit, 
one  hemisphere  would  receive  unbroken  sunshine,  the 
other  remaining  in  perpetual  darkness  and  cold.  In  this 
condition,  in  place  of  an  equatorial  line  we  should  have 
an  equatorial  point  at  the  pole  nearest  the  sun;  thence  the 
temperatures  would  grade  away  to  the  present  equator, 
beyond  which  half  the  earth  would  be  in  more  refrigerat- 
ing condition  than  are  the  poles  at  the  present  day.  In 
considering  the  movements  of  our  planet,  we  shall  see  that 
no  great  changes  in  the  position  of  the  polar  axis  can  have 
taken  place.  On  this  account  the  suggested  alterations  of 
the  axis  should  not  be  taken  as  other  than  imaginary 
changes. 

It  is  easy  to  see  that  with  a  circular  orbit  and  with 
an  inclined  axis  winter  and  summer  would  normally  come 
always  at  the  same  point  in  the  orbit,  and  that  these  sea- 
sons would  be  of  perfectly  even  length.  But,  as  we  have 
before  noted,  the  earth's  path  around  the  sun  is  in  its  form 
greatly  affected  by  the  attractions  which  are  exercised  by 
the  neighbouring  planets,  principally  by  those  great  spheres 
which  lie  in  the  realm  without  its  orbit,  Jupiter  and 
Saturn.    When  these  attracting  bodies,  as  is  the  case  from 


60  OUTLINES  OP  THE  EARTH'S  HISTORY. 

time  to  time,  though  at  long  intervals,  are  brought  to- 
gether somewhere  near  to  that  part  of  the  solar  system 
in  which  the  earth  is  moving  around  the  sun,  they  draw 
our  planet  toward  them,  and  so  make  its  path  very  ellip- 
tical. When,  however,  they  are  so  distributed  that  their 
pulling  actions  neutralize  each  other,  the  orbit  returns 
more  nearly  to  a  circular  form.  The  range  in  its  eccen- 
tricity which  can  be  brought  about  by  these  alterations  is 
very  great.  When  the  path  is  most  nearly  circular,  the 
difference  in  the  major  and  minor  axis  may  amount  to  as 
little  as  about  five  hundred  thousand  miles,  or  about  one 
one  hundred  and  eighty-sixth  of  its  average  diameter. 
When  the  variation  is  greatest  the  difference  in  these 
measurements  may  be  as  much  as  near  thirteen  million 
miles,  or  about  one  seventh  of  the  mean  width  of  the  orbit. 

The  first  and  most  evident  effect  arising  from  these 
changes  of  the  orbit  comes  from  the  difference  in  the 
amount  of  heat  which  the  earth  may  receive  according  as 
it  is  nearer  or  farther  from  the  sun.  As  in  the  case  of 
other  fires,  the  nearer  a  body  is  to  it  the  larger  the  share 
of  light  and  heat  which  it  will  receive.  In  an  orbit  made 
elliptical  by  the  planetary  attraction  the  sun  necessarily 
occupies  one  of  the  foci  of  the  ellipse.  The  result  is,  of 
course,  that  the  side  of  the  earth  which  is  toward  the  sun,' 
while  it  is  thus  brought  the  nearer  to  the  luminary,  re- 
ceives more  energy  in  the  form  of  light  and  heat  than  come 
to  any  part  which  is  exposed  when  the  spheres  are  farther 
away  from  each  other  in  the  other  part  of  the  orbit.  Com- 
putations clearly  show  that  the  total  amount  of  heat  and 
the  attendant  light  which  the  earth  receives  in  a  year  is 
not  affected  by  these  changes  in  the  form  of  its  path.  "\Yhile 
it  is  true  that  it  receives  heat  more  rapidly  in  the  half  of 
the  ellipse  which  is  nearest  the  source  of  the  inundation, 
it  obtains  less  while  it  is  farther  away,  and  these  two  varia- 
tions just  balance  each  other. 

Although  the  alterations  in  the  eccentricity  of  its  orbit 
do  not  vary  the  annual  supply  of  heat  which  the  earth  re- 


THE   STELLAR  REALM.  61 

ceives,  they  are  capable  of  changing  the  character  of  the 
seasons,  and  this  in  the  way  which  we  will  now  endeavour 
to  set  forth,  though  we  must  do  it  at  the  cost  of  consider- 
able attention  on  the  part  of  the  reader,  for  the  facts  are 
somewhat  complicated.  In  the  first  place,  we  must  note 
that  the  ellipticity  of  the  earth's  orbit  is  not  developed  on 
fixed  lines,  but  is  endlessly  varied,  as  we  can  readily  im- 
agine it  would  be  for  the  reason  that  its  form  depends  upon 
the  wandering  of  the  outer  planetary  spheres  which  pull 
the  earth  about.  The  longer  axis  of  the  ellipse  is  itself 
in  constant  motion  in  the  direction  in  which  the  earth 
travels.  This  movement  is  slow,  and  at  an  irregular  rate. 
It  is  easy  to  see  that  the  effect  of  this  action,  which  is  called 
the  revolution  of  the  apsides,  or,  as  the  word  means,  the 
movement  of  the  poles  of  the  ellipse,  is  to  bring  the  earth, 
when  a  given  hemisphere  is  turned  toward  the  sun,  some- 
times in  the  part  of  the  orbit  which  is  nearest  the  source 
of  light  and  heat,  and  sometimes  farther  away.  It  may 
thus  well  come  about  that  at  one  time  the  summer  season 
of  a  hemisphere  arrives  when  it  is  nearest  the  sun,  so  that 
the  season,  though  hot,  will  be  very  short,  while  at  another 
time  the  same  season  will  arrive  when  the  earth  is  farthest 
from  the  sun,  and  receives  much  less  heat,  which  would 
tend  to  make  a  long  and  relatively  cool  summer.  The 
reason  for  the  difference  in  length  of  the  seasons  is  to  be 
found  in  the  relative  swiftness  of  the  earth's  revolution 
when  it  is  nearest  the  sun,  and  the  slowness  when  it  is  far- 
ther away. 

There  is  a  further  complication  arising  from  that  curi- 
ous phenomenon  called  the  precession  of  the  equinoxes, 
which  has  to  be  taken  into  account  before  we  can  suffi- 
ciently comprehend  the  effect  of  the  varying  eccentricity 
of  the  orbit  on  the  earth's  seasons.  To  understand  this 
feature  of  precession  we  should  first  note  that  it  means 
that  each  year  the  change  from  the  winter  to  the  summer — 
or,  as  we  phrase  it,  the  passage  of  the  equinoctial  line — 
occurs  a  little  sooner  than  the  year  before.    The  cause  of 


62  OUTLINES   OF   THE  EARTH'S  HISTORY. 

tliis  is  to  be  found  in  the  attraction  which  the  heavenly 
bodies,  practically  altogether  the  moon,  exercises  on 
the  equatorial  protuberance  of  the  earth.  We  know  that 
the  diameter  of  our  sphere  at  the  equator  is,  on  the  aver- 
age, something  more  than  twenty-six  miles  greater  than 
it  is  through  the  poles.  We  know,  furthermore,  that 
the  position  of  the  moon  in  relation  to  the  earth  is  such 
that  it  causes  the  attraction  on  one  half  of  this  protuber- 
ance to  be  greater  than  it  is  upon  the  other.  We  readily 
perceive  that  this  action  will  cause  the  polar  axis  to  make 
a  certain  revolution,  or,  what  comes  to  the  same  thing, 
that  the  plane  of  the  equator  will  constantly  be  altering  its 
position.  Now,  as  the  equinoctial  points  in  the  orbit  de- 
pend for  their  position  upon  the  attitude  of  the  equatorial 
plane,  we  can  conceive  that  the  effect  is  a  change  in  posi- 
tion of  the  place  in  that  orbit  where  summer  and  winter 
begin.  The  actual  result  is  to  bring  the  seasonal  points 
backward,  step  by  step,  through  the  orbit  in  a  regular 
measure  until  in  twenty-two  thousand  five  hundred  years 
they  return  to  the  place  where  they  were  before.  This 
cycle  of  change  was  of  old  called  the  Annus  Magnus,  or 
great  year. 

If  the  earth's  orbit  were  an  ellipse,  the  major  axis  of 
which  remained  in  the  same  position,  we  could  readily 
reckon  all  the  effects  which  arise  from  the  variations  of 
the  great  year.  But  this  ellipse  is  ever  changing  in  form, 
and  in  the  measure  of  its  departure  from  a  circle  the  effects 
on  the  seasons  distributed  over  a  great  period  of  time  are 
exceedingly  irregular.  Now  and  then,  at  intervals  of  hun- 
dreds of  thousands  or  millions  of  years,  the  orbit  becomes 
very  elliptical;  then  again  for  long  periods  it  may  in  form 
approach  a  circle.  When  in  the  state  of  extreme  ellipticity, 
the  precession  of  the  equinoxes  will  cause  the  hemispheres 
in  turn  each  to  have  their  winter  and  summer  alternately 
near  and  far  from  the  sun.  It  is  easily  seen  that  when 
the  summer  season  comes  to  a  hemisphere  in  the  part  of 
the  orbit  which  is  then  nearest  the  sun  the  period  will  be 


THE  STELLAR  REALM.  63 

very  hot.  When  the  summer  came  farthest  from  the  sun 
that  part  of  the  year  would  have  the  temperature  mitigated 
by  its  removal  to  a  greater  distance  from  the  source  of  heat. 
A  corresponding  effect  would  be  produced  in  the  winter 
season.  As  long  as  the  orbit  remained  eccentric  the  tend- 
ency would  be  to  give  alternately  intense  seasons  to  each 
hemisphere  through  periods  of  about  twelve  thousand  years, 
the  other  hemisphere  having  at  the  same  time  a  relatively 
slight  variation  in  the  summer  and  winter. 

At  first  sight  it  may  seem  to  the  reader  that  these 
studies  we  have  just  been  making  in  matters  concerning 
the  shape  of  the  orbit  and  the  attendant  circumstances 
which  regulate  the  seasons  were  of  no  very  great  conse- 
quence; but,  in  the  opinion  of  some  students  of  climate, 
we  are  to  look  to  these  processes  for  an  explanation  of  cer- 
tain climatal  changes  on  the  earth,  including  the  Glacial 
periods,  accidents  which  have  had  the  utmost  importance 
in  the  history  of  man,  as  well  as  of  all  the  other  life  of  the 
planet. 

It  is  now  time  to  give  some  account  as  to  what  is  known 
concerning  the  general  conditions  of  the  solar  bodies — the 
planets  and  satellites  of  our  own  celestial  group.  For  our 
purpose  we  need  attend  only  to  the  general  physical  state 
of  these  orbs  so  far  as  it  is  known  to  us  by  the  studies  of 
astronomers.  The  nearest  planet  to  the  sun  is  Mercury. 
This  little  sphere,  less  than  half  the  diameter  of  our  earth, 
is  so  close  to  the  sun  that  even  when  most  favourably 
placed  for  observation  it  is  visible  for  but  a  few  minutes 
before  sunrise  and  after  sunset.  Although  it  may  without 
much  difficulty  be  found  by  the  ordinary  eye,  very  few 
people  have  ever  seen  it.  To  the  telescope  when  it  is  in 
the  full  moon  state  it  appears  as  a  brilliant  disk;  it  is  held 
by  most  astronomers  that  the  surface  which  we  see  is  made 
up  altogether  of  clouds,  but  this,  as  most  else  that  has 
been  stated  concerning  this  planet,  is  doubtful.  The 
sphere  is  so  near  to  the  sun  that  if  it  were  possessed  of 
water  it   would   inevitably  bear   an   atmosphere   full   of 


64  OUTLINES  OF  THE  EARTH'S  HISTORY. 

vapour.  Under  any  conceivable  conditions  of  a  planet 
placed  as  Mercury  is,  provided  it  had  an  atmosphere  to 
retain  the  heat,  its  temperature  would  necessarily  be  very 
high.  Life  as  we  know  it  could  not  well  exist  upon  such 
a  sphere. 

Next  beyond  Mercury  is  Venus,  a  sphere  only  a  little 
less  in  diameter  than  the  earth.  Of  this  sphere  we  know 
more  than  we  do  of  Mercury,  for  the  reason  that  it  is  far- 
ther from  the  sun  and  so  appears  in  the  darkened  sky. 
Most  astronomers  hold  that  the  surface  of  this  planet 
apparently  is  almost  completely  and  continually  hidden 
from  us  by  what  appears  to  be  a  dense  cloud  envelope, 
through  which  from  time  to  time  certain  spots  appear  of 
a  dark  colour.  These,  it  is  claimed,  retain  their  place  in 
a  permanent  way;  it  is,  indeed,  by  observing  them  that 
the  rotation  period  of  the  planet  has,  according  to  some 
observers,  been  determined.  It  therefore  seems  likely  that 
these  spots  are  the  summits  of  mountains,  which,  like  many 
of  our  own  earth,  rise  above  the  cloud  level. 

Eecent  observations  on  Venus  made  by  Mr.  Percival 
Lowell  appear  to  show  that  the  previous  determinations  of 
the  rotation  of  that  planet,  as  well  as  regards  its  cloud  wrap, 
are  in  error.  According  to  these  observations,  the  sphere 
moves  about  the  sun,  always  keeping  the  same  side  turned 
toward  the  solar  centre,  just  as  the  moon  does  in  its  mo- 
tion around  the  earth.  Moreover,  Mr.  Lowell  has  failed 
to  discover  any  traces  of  clouds  upon  the  surface  of  the 
planet.  As  yet  these  results  have  not  been  verified  by  the 
work  of  other  astronomers;  resting,  however,  as  they  do 
on  studies  made  with  an  excellent  telescope  and  in  the  very 
translucent  and  steady  air  of  the  Flagstaff  Station,  they  are 
more  likely  to  be  correct  than  those  obtained  by  other 
students.  If  it  be  true  that  Venus  does  not  turn  upon  its 
axis,  such  is  likely  to  be  the  case  also  with  the  planet  Mer- 
cury. 

Next  in  the  series  of  the  planets  is  our  own  earth.  As 
the  details  of  this  planet  are  to  occupy  us  during  nearly 


THE  STELLAR  HEALM.  65 

all  the  remainder  of  this  work,  we  shall  for  the  present 
pass  it  by. 

Beyond  the  earth  we  pass  first  to  the  planet  Mars,  a 
sphere  which  has  already  revealed  to  us  much  concerning 
its  peculiarities  of  form  and  physical  state,  and  which  is 
likely  in  the  future  to  give  more  information  than  we  shall 
obtain  from  any  other  of  our  companions  in  space,  except 
perhaps  the  moon.  Mars  is  not  only  nearer  to  us  than  any 
other  planet,  but  it  is  so  placed  that  it  receives  the  light 
of  the  sun  under  favourable  conditions  for  our  vision. 
Moreover,  its  sky  appears  to  be  generally  almost  cloudless, 
so  that  when  in  its  orbital  course  the  sphere  is  nearest  our 
earth  it  is  under  favourable  conditions  for  telescopic  ob- 
servation. At  such  times  there  is  revealed  to  the  astrono- 
mer a  surface  which  is  covered  with  an  amazing  number 
of  shadings  and  markings  which  as  yet  have  been  incom- 
pletely interpreted.  The  faint  nature  of  these  indications 
has  led  to  very  contradictory  statements  as  to  their  form; 
no  two  maps  which  have  been  drawn  agree  except  in  their 
generalities.  There  is  reason  to  believe  that  Mars  has  an 
atmosphere;  this  is  shown  by  the  fact  that  in  the  appro- 
priate season  the  region  about  either  pole  is  covered  by  a 
white  coating,  presumably  snow.  This  covering  extends 
rather  less  far  toward  the  planet's  equator  than  does  the 
snow  sheet  on  our  continents.  Taking  into  account  the 
colour  of  the  coating,  and  the  fact  that  it  disappears  when 
the  summer  season  comes  to  the  hemisphere  in  which  it 
was  formed,  we  are,  in  fact,  forced  to  believe  that  the  der 
posit  is  frozen  water,  though  it  has  been  suggested  that 
it  may  be  frozen  carbonic  acid.  Taken  in  connection  with 
what  we  have  shortly  to  note  concerning  the  apparent  seas 
of  this  sphere,  the  presumption  is  overwhelmingly  to  the 
effect  that  Mars  has  seasons  not  unlike  our  own. 

The  existence  of  snow  on  any  sphere  may  safely  be 
taken  as  evidence  that  there  is  an  atmosphere.  In  the 
case  of  Mars,  this  supposition  is  borne  out  by  the  appcnr- 
ance  of  its  surface.    The  ruddy  light  which  it  sends  lack 


6Q 


OUTLINES  OF  THE  EARTHS  HISTORY. 


to  us,  and  the  appearance  on  the  margin  of  the  sphere, 
which  is  somewhat  dim,  appears  to  indicate  that  its  at- 
mosphere is  dense.  In  fact,  the  existence  of  an  atmosphere 
much  denser  than  that  of  our  own  earth  appears  to  be 
demanded  by  the  fact  that  the  temperatures  are  such  as  to 
permit  the  coming  and  going  of  snow.  It  is  well  known 
that  the  temperature  of  any  point  on  the  earth,  other 
things  being  equal,  is  proportionate  to  the  depth  of  atmos- 
phere above  its  surface.  If  Mars  had  no  more  air  over  its 
surface  than  has  an  equal  area  of  the  earth,  it  would  re- 
main at  a  temperature  so  low  that  such  seasonal  changes 
as  we  have  observed  could  not  take  place.  The  planet 
receives  one  third  less  heat  than  an  equal  area  of  the 
earth,  and  its  likeness  to  our  own  temperature,  if  such 

exists,  is  doubtless 
brought  about  by 
the  greater  den- 
sity of  its  atmos- 
phere, that  serves 
to  retain  the  heat 
w^hich  comes  upon 
its  surface.  The 
manner  in  which 
this  is  effected 
will  be  set  forth 
in  the  study  of 
the  earth's  atmos- 
phere. 

As  is  shown 
by  the  maps  of 
Mars,  the  surface 
is  occupied  by 
shadings  which 
seem  to  indicate  the  existence  of  w^ater  and  lands.  Those 
portions  of  the  area  which  are  taken  to  be  land  are  very 
much  divided  by  what  appear  to  be  narrow  seas.  The 
general  geographic  conditions  differ  much  from  those  of 


Fig.  4.— Mars,  August  27,  1892  (Guiot),  the 
white  patch  is  the  supposed  Polar  Snow  Cap. 


THE  STELLAR  REALM.  67 

our  own  sphere  in  that  the  parts  of  the  planet  about  the 
water  level  are  not  grouped  in  great  continents,  and  there 
are  no  large  oceans.  The  only  likeness  to  the  conditions 
of  our  earth  which  we  can  perceive  is  in  a  general  point- 
ing of  the  somewhat  triangular  masses  of  what  appears 
to  be  land  toward  one  pole.  As  a  whole,  the  conditions  of 
the  Martial  lands  and  seas  as  regards  their  form,  at  least, 
is  more  like  that  of  Europe  than  that  of  any  other  part 
of  the  earth's  surface.  Europe  in  the  early  Tertiary  times 
had  a  configuration  even  more  like  that  of  Mars  than  it 
exhibits  at  present,  for  in  that  period  the  land  was  very 
much  more  divided  than  it  now  is. 

If  the  lands  of  Mars  are  framed  as  are  those  of  our 
own  earth,  there  should  be  ridges  of  mountains  constitut- 
ing what  we  may  term  the  backbones  of  the  continent. 
As  yet  such  have  not  been  discerned,  which  may  be  due  to 
the  fact  that  they  have  not  been  carefully  looked  for.  The 
only  peculiar  physical  features  which  have  as  yet  been 
discerned  on  the  lands  of  Mars  are  certain  long,  straight, 
rather  narrow  crevicelike  openings,  which  have  received 
the  name  of  "  canals."  These  features  are  very  indistinct, 
and  are  just  on  the  limit  of  visibility.  As  yet  they  have 
been  carefully  observed  by  but  few  students,  so  that  their 
features  are  not  yet  well  recorded;  as  far  as  we  know 
them,  these  fissures  have  no  likeness  in  the  existing  condi- 
tions of  our  earth.  It  is  diflficult  to  understand  how  they 
are  formed  or  preserved  on  a  surface  which  is  evidently 
subjected  to  rainfalls. 

It  will  require  much  more  efficient  telescopes  than  we 
now  have  before  it  will  be  possible  to  begin  any  satisfac- 
tory study  on  the  geography  of  this  marvellous  planet. 
We  can  not  hope  as  yet  to  obtain  any  indications  as  to  the 
details  of  its  structure;  we  can  not  see  closely  enough  to 
determine  whether  rivers  exist,  or  whether  there  is  a  coat- 
ing which  we  may  interpret  as  vegetation,  changing  its 
hues  in  the  different  seasons  of  the  year._  An  advance  in 
our  instruments  of  research  during  the  coming  century, 


68  OUTLINES  OF  THE  EARTH'S  HISTORY. 

if  made  with  the  same  speed  as  during  the  last,  will  per- 
haps enable  us  to  interpret  the  nature  of  this  neighbour, 
and  thereby  to  extend  the  conception  of  planetary  histories 
which  we  derive  from  our  own  earth. 

Beyond  Mars  we  find  one  of  the  most  singular  features 
of  our  solar  system  in  a  group  of  small  planetary  bodies, 
the  number  of  which  now  known  amounts  to  some  two 
hundred,  and  the  total  may  be  far  greater.  These  bodies 
are  evidently  all  small;  it  is  doubtful  if  the  largest  is  three 
hundred  and  the  smaller  more  than  twenty  miles  in  diame- 


Fia.  5. — Comparative  Sizes  of  the  Planets  (Chambers). 

ter.    So  far  as  it  has  been  determined  by  the  effect  of  their 
aggregate  jn^iss  in  attracting  the  other  spheres,  they  would. 


THE  STELLAR  REALM.  69 

if  put  together,  make  a  sphere  far  less  in  diameter  than  our 
earth,  perhaps  not  more  than  five  hundred  miles  through. 
The  forms  of  these  asteroids  is  as  yet  unknown;  we  there- 
fore can  not  determine  whether  their  shapes  are  spheroidal, 
as  are  those  of  the  other  planets,  or  whether  they  are  angu- 
lar bits  like  the  meteorites.  We  are  thus  not  in  a  position 
to  conjecture  whether  their  independence  began  when  the 
nebulous  matter  of  the  ring  to  which  they  belonged  was  in 
process  of  consolidation,  or  whether,  after  the  aggregation 
of  the  sphere  was  accomplished,  and  the  matter  solidified, 
the  mass  was  broken  into  bits  in  some  way  which  we  can 
not  yet  conceive.  It  has  been  conjectured  that  such  a  solid 
sphere  might  have  been  driven  asunder  by  a  collision  with 
some  wandering  celestial  body;  but  all  we  can  conceive 
of  such  actions  leads  us  to  suppose  that  a  blow  of  this  na- 
ture would  tend  to  melt  or  convert  materials  subjected  to 
it  into  the  state  of  vapour,  rather  than  to  drive  them  asunder 
in  the  manner  of  an  explosion. 

The  four  planets  which  lie  beyond  the  asteroids  give 
us  relatively  little  information  concerning  their  physical 
condition,  though  they  afford  a  wide  field  for  the  philo- 
sophic imagination.  From  this  point  of  view  the  reader  is 
advised  to  consult  the  writings  of  the  late  R.  A.  Proctor, 
who  has  brought  to  the  task  of  interpreting  the  planetary 
conditions  the  skill  of  a  well-trained  astronomer  and  a 
remarkable  constructive  imagination. 

The  planet  Jupiter,  by  far  the  largest  of  the  children 
of  the  sun,  appears  to  be  still  in  a  state  where  its  internal 
heat  has  not  so  far  escaped  that  the  surface  has  cooled 
down  in  the  manner  of  our  earth.  What  appear  to  be 
good  observations  show  that  the  equatorial  part  of  its  area, 
at  least,  still  glows  from  its  own  heat.  The  sphere  is  cloud- 
wrapped,  but  it  is  doubtful  whether  the  envelope  be  of 
watery  vapour;  it  is,  indeed,  quite  possible  that  besides  such 
vapour  it  may  contain  some  part  of  the  many  substances 
which  occupy  the  atmosphere  of  the  sun.  If  the  Jovian 
sphere  were  no  larger  than  the  earth,  it  would,  on  account 


70  OUTLINES  OF  THE  EARTH'S  HISTORY. 

of  its  greater  age,  long  ago  have  parted  with  its  heat;  but 
on  account  of  its  great  size  it  has  been  able,  notwithstand- 
ing its  antiquity,  to  retain  a  measure  of  temperature  which 
has  long  since  passed  away  from  our  earth. 

In  the  case  of  Saturn,  the  cloud  bands  are  somewhat 
less  visible  than  on  Jupiter,  but  there  is  reason  to  suppose 
in  this,  as  in  the  last-named  planet,  that  we  do  not  behold 
the  more  solid  surface  of  the  sphere,  but  see  only  a  cloud 
wrap,  which  is  probably  due  rather  to  the  heat  of  the  sphere 
itself  than  to  that  which  comes  to  it  from  the  sun.  At 
the  distance  of  Saturn  from  the  centre  of  the  solar  system 
a  given  area  of  surface  receives  less  than  one  ninetieth  of 
the  sun^s  heat  as  compared  with  the  earth;  therefore  we 
can  not  conceive  that  any  density  of  the  atmosphere  what- 
ever would  suffice  to  hold  in  enough  temperature  to  pro- 
duce ordinary  clouds.  Moreover,  from  time  to  time  bright 
spots  appear  on  the  surface  of  the  planet,  w^hich  must  be 
due  to  some  form  of  eruptions  from  its  interior. 

Beyond  Saturn  the  two  planets  Uranus  and  Neptune, 
which  occupy  the  outer  part  of  the  solar  system,  are  so 
remote  that  even  our  best  telescopes  discern  little  more 
than  their  presence,  and  the  fact  that  they  have  attendant 
moons. 

From  the  point  of  view  of  astronomical  science,  the 
outermost  planet  Neptune,  of  peculiar  interest  for  the 
reason  that  it  was,  as  we  may  say,  discovered  by  computa- 
tion. Astronomers  had  for  many  years  remarked  the  fact 
that  the  next  inner  planetary  sphere  exhibited  peculiari- 
ties in  its  orbit  which  could  only  be  accounted  for  on  the 
supposition  that  it  was  subjected  to  the  attraction  of  an- 
other wandering  body  which  had  escaped  observation.  By 
skilful  computation  the  place  in  the  heavens  in  which 
this  disturbing  element  lay  was  so  accurately  determined 
that  when  the  telescope  was  turned  to  the  given  field  a 
brief  study  revealed  the  planet.  Nothing  else  in  the  his- 
tory of  the  science  of  astronomy,  unless  it  be  the  computa- 
tion of  eclipses,  so  clearly  and  popularly  shows  the  accuracy 


THE  STELLAR  REALM.  71 

of  the  methods  by  which  the  work  of  that  science  may  be 
done. 

As  we  shall  see  hereafter,  in  the  chapters  which  are  de- 
voted to  terrestrial  phenomena,  the  physical  condition  of 
the  sun  determines  the  course  of  all  the  more  important 
events  which  take  place  on  the  surface  of  the  earth.  It  is 
therefore  fit  that  in  this  preliminary  study  of  the  celes- 
tial bodies,  which  is  especially  designed  to  make  the  earth 
more  interpretable  to  us,  we  should  give  a  somewhat  spe- 
cial attention  to  what  is  known  under  the  title  of  "  Solar 
Physics." 

The  reader  has  already  been  told  that  the  sun  is  one 
of  many  million  similar  bodies  which  exist  in  space,  and, 
furthermore,  that  these  aggregations  of  matter  have  been 
developed  from  an  original  nebulous  condition.  The  facts 
indicate  that  the  natural  history  of  the  sun,  as  well  as 
that  of  its  attendant  spheres,  exhibits  three  momentous 
stages:  First,  that  of  vapour;  second,  that  of  igneous 
fluidity;  third,  that  in  which  the  sphere  is  so  far  con- 
gealed that  it  becomes  dark.  Neither  of  these  states  is 
sharply  separated  from  the  other;  a  mass  may  be  partly 
nebulous  and  partly  fluid;  even  when  it  has  been  con- 
verted into  fluid,  or  possibly  into  the  solid  state,  it  may 
still  retain  on  the  exterior  some  share  of  its  original  vapor- 
ous condition.  In  our  sun  the  concentration  has  long  since 
passed  beyond  the  limits  of  the  nebulous  state;  the  last 
of  the  successively  developed  rings  has  broken,  and  has 
formed  itself  into  the  smallest  of  the  planets,  which  by  its 
distance  from  the  sun  seems  to  indicate  that  the  process 
of  division  by  rings  long  ago  attained  in  our  solar  system 
its  end,  the  remainder  of  its  nebulous  material  concentrat- 
ing on  its  centre  without  sign  of  any  remaining  tendency 
to  produce  these  planet-making  circles. 


t2  OUTLINES  OF  THE  EARTH'S  HISTORY. 

The  Constitution  of  the  Sun. 

Before  the  use  of  the  telescope  in  astronomical  work, 
which  was  begun  by  the  illustrious  Galileo  in  1608, 
astronomers  were  unable  to  approach  the  problem  of  the 
structure  of  the  sun.  They  could  discern  no  more  than 
can  be  seen  by  any  one  who  looks  at  the  great  sphere 
through  a  bit  of  smoked  glass,  as  we  know  this  reveals  a 
disklike  body  of  very  uniform  appearance.  The  only  varia- 
tion in  this  simple  aspect  occurs  at  the  time  of  a  total 
eclipse,  when  for  a  minute  or  two  the  moon  hides  the  whole 
body  of  the  sun.  On  such  occasions  even  the  unaided  eye 
can  see  that  there  is  about  the  sphere  a  broad,  rather  bright 
field,  of  an  aspect  like  a  very  thin  cloud  or  fog,  which 
rises  in  streamerlike  projections  at  points  to  a  quarter  of  a 
million  miles  or  more  above  the  surface  of  the  sphere.  The 
appearance  of  this  shining  field,  which  is  called  the  corona, 
reminds  one  of  the  aurora  which  glows  in  the  region  about 
either  pole  of  the  earth. 

One  of  the  first  results  of  the  invention  of  the  telescope 
was  the  revelation  of  the  curious  dark  objects  on  the  sun's 
disk,  known  by  the  name  of  spots  from  the  time  of  their 
discovery,  or,  at  least,  from  the  time  when  it  was  clearly 
perceived  that  they  were  not  planets,  but  really  on  the 
solar  body.  The  interest  in  the  constitution  of  the  sphere 
has  increased  during  the  last  fifty  years.  This  interest  has 
rapidly  grown  until  at  the  present  time  a  vast  body  of 
learning  has  been  gathered  for  the  solution  of  the  many 
problems  concerning  the  centre  of  our  system.  As  yet 
there  is  great  divergence  in  the  views  of  astronomers  as 
to  the  interpretation  of  their  observations,  but  certain 
points  of  great  general  interest  have  been  tolerably  well 
determined.  These  may  be  briefly  set  forth  by  an  account 
of  what  would  meet  the  eye  if  an  observer  were  able  to 
pass  from  the  surface  of  the  earth  to  the  central  part  of 
the  sun. 

In  passing  from  the  earth  to  a  point  about  a  quarter 


THE  STELLAR  REALM.  73 

of  a  million  miles  from  the  sun's  surface — a  distance  about 
that  of  the  moon  from  our  sphere — the  observer  would 
traverse  the  uniformly  empty  spaces  of  the  heavens,  where, 
but  for  the  rare  chance  of  a  passing  meteorite  or  comet, 
there  would  be  nothing  that  we  term  matter.  x\rriving  at 
a  point  some  two  or  three  hundred  thousand  miles  from 
the  body  of  the  sun,  he  would  enter  the  realm  of  the 
corona;  here  he  would  find  scattered  particles  of  matter, 
the  bits  so  far  apart  that  there  would  perhaps  be  not  more 
than  one  or  two  in  the  cubic  mile;  yet,  as  they  would 
glow  intensely  in  the  central  light,  they  would  be  sufficient 
to  give  the  illumination  which  is  visible  in  an  eclipse. 
These  particles  are  most  likely  driven  up  from  the  sun 
by  some  electrical  action,  and  are  constantly  in  motion, 
much  as  are  the  streamers  of  the  aurora. 

Below  the  corona  and  sharply  separated  from  it  the 
observer  finds  another  body  of  very  dense  vapour,  which 
is  termed  the  chromosphere,  and  which  has  been  regarded 
as  the  atmosphere  of  the  sun.  This  layer  is  probably  sev- 
eral thousand  miles  thick.  From  the  manner  in  which  it 
moves,  in  the  way  the  air  of  our  own  planet  does  in  great 
storms,  it  is  not  easy  to  believe  that  it  is  a  fluid,  yet  its 
sharply  defined  upper  surface  leads  us  to  suppose  that 
it  can  not  well  be  a  mere  mass  of  vapour.  The  spectro- 
scope shows  us  that  this  chromosphere  contains  in  the  state 
of  vapour  a  number  of  metallic  substances,  such  as  iron 
and  magnesium.  To  an  observer  who  could  behold  this 
envelope  of  the  sun  from  the  distance  at  which  we  see  the 
moon,  the  spectacle  would  be  more  magnificent  than  the 
imagination,  guided  by  the  sight  of  all  the  relatively  trifling 
fractures  of  our  earth,  can  possibly  conceive.  From  the 
surface  of  the  fiery  sea  vast  uprushes  of  heated  matter 
rise  to  the  height  of  two  or  three  hundred  thousand  miles, 
and  then  fall  back  upon  its  surface.  These  jets  of  heated 
matter  have  the  aspect  of  flames,  but  they  would  not  be 
such  in  fact,  for  the  materials  are  not  burning,  but  merely 
kept  at  a  high  temperature  by  the  heat  of  the  great  sphere 


74  OUTLINES  OF  THE  EARTH'S  HISTORY. 

beneath.  They  spring  up  with  such  energy  that  they  at 
times  move  with  a  speed  of  one  hundred  and  fifty  miles  a 
second,  or  at  a  rate  which  is  attained  by  no  other  matter  in 
the  visible  universe,  except  that  strange,  wandering  star 
known  to  astronomers  as  "  Grombridge,  1830,"  which  is 
traversing  the  firmament  with  a  speed  of  not  less  than  two 
hundred  miles  a  second. 

BeloAv  the  chromosphere  is  the  photosphere,  the  lower 
envelope  of  the  sun,  if  it  be  not  indeed  the  body  of  the 
sphere  itself;  from  this  comes  the  light  and  heat  of  the 
mass.  This,  too,  can  not  well  be  a  firm-set  mass,  for  the 
reason  that  the  spots  appear  to  form  in  and  move  over  it. 
It  may  be  regarded  as  an  extremely  dense  mass  of  gas,  so 
weighed  down  by  the  vast  attraction  of  the  great  sphere 
below  it  that  it  is  in  effect  a  fluid.  The  near-at-hand  ob- 
server would  doubtless  find  this  photosphere,  as  it  appears 
in  the  telescope,  to  be  sharply  separated  from  the  thinner 
and  more  vaporous  envelopes — the  chromosphere  and  the 
corona — which  are,  indeed,  so  thin  that  they  are  invisible 
even  with  the  telescope,  except  when  the  full  blaze  of  the 
sun  is  cut  off  in  a  total  eclipse.  The  fact  that  the  photo- 
sphere, except  when  broken  by  the  so-called  spots,  lies  like 
a  great  smooth  sea,  with  no  parts  which  lie  above  the  gen- 
eral line,  shows  that  it  has  a  very  different  structure  from 
the  envelope  which  lies  upon  it.  If  they  were  both  vapor- 
ous, there  would  be  a  gradation  between  them. 

On  the  surface  of  the  photosphere,  almost  altogether 
within  thirty  degrees  of  the  equator  of  the  sun,  a  field  cor- 
responding approximately  to  the  tropical  belt  of  the  earth, 
there  appear  from  time  to  time  the  curious  disturbances 
which  are  termed  spots.  These  appear  to  be  uprushes  of 
matter  in  the  gaseous  state,  the  upward  movement  being 
upon  the  margins  of  the  field  and  a  downward  motion  tak- 
ing place  in  the  middle  of  the  irregular  opening,  which 
is  darkened  in  its  central  part,  thus  giving  it,  when  seen 
by  an  ordinary  telescope,  the  aspect  of  a  black  patch  on 
the  glowing  surface.    These  spots,  which  are  from  some 


THE  STELLAR  REALM. 


75 


hundred  to  some  thousand  miles  in  diameter,  may  enduie 
for  months  before  they  fade  away.  It  is  clear  that  they  are 
most  abundant  at  intervals  of  about  eleven  years,  the  last 
period  of  abundance  being  in  1893.  The  next  to  come 
may  thus  be  expected  in  1904:.  In  the  times  of  least  spot- 
ting more  than  half  the  days  of  a  year  may  pass  without  the 
surface  of  the  photosphere  being  broken,  while  in  periods 
of  plenty  no  day  in  the  year  is  likely  to  fail  to  show  them. 

It  is  doubtful  if  the  closest  seeing  would  reveal  the 
cause  of  the  solar  spots.  The  studies  of  the  physicists  who 
have  devoted  the  most  skill  to  the  matter  show  little  more 
than  that  they  are  tumults  in  the  photosphere,  attended 

by  an  uprush  of  va- 
pours, in  which  iron 
and  other  metals  ex- 
ist; but  whether  these 
movements  are  due  to 
outbreaks  from  the 
deeper  parts  of  the 
sun  or  to  some  ac- 
tion like  the  whirling 
storms  of  the  earth's 
atmosphere  is  uncer- 
tain. It  is  also  uncer- 
tain what  effect  these 
convulsions  of  the  sun 
have  on  the  amount 
of  the  heat  and  light 
which  is  poured  forth  from  the  orb.  The  common  opinion 
that  the  sun-spot  years  are  the  hottest  is  not  yet  fully 
verified. 

Below  the  photosphere  lies  the  vast  unknown  mass  of 
the  unseen  solar  realm.  It  was  at  one  time  supposed  that 
the  dark  colour  of  the  spots  was  due  to  the  fact  that  the 
photosphere  was  broken  through  in  those  spaces,  and  that  we 
looked  down  through  them  upon  the  surface  of  the  slightly 
illuminated  central  part  of  the  sphere.    This  view  is  unten- 


FiG.  G. — Ordinary  Sun-spot,  June 
22,  1885. 


7G  OUTLINES  OF  THE  EARTH'S  HISTORY. 

able,  and  in  its  place  we  have  to  assume  that  for  the  eight 
hundred  and  sixty  thousand  miles  of  its  diameter  the 
sun  is  composed  of  matter  such  as  is  found  in  our  earth, 
but  throughout  in  a  state  of  heat  which  vastly  exceeds  that 
known  on  or  in  our  planet.  Owing  to  its  heat,  this  matter 
is  possibly  not  in  either  the  solid  or  the  fluid  state,  but  in 
that  of  very  compressed  gases,  which  are  kept  from  be- 
coming solid  or  even  fluid  by  the  very  high  temperature 
which  exists  in  them.  This  view  is  apparently  supported  by 
the  fact  that,  while  the  pressure  upon  its  matter  is  twenty- 
seven  times  greater  in  the  sun  than  it  is  in  the  earth,  the 
weight  of  the  whole  mass  is  less  than  we  should  expect 
under  these  conditions. 

As  for  the  temperature  of  the  sun,  we  only  know  that 
it  is  hot  enough  to  turn  the  metals  into  gases  in  the  man- 
ner in  which  this  is  done  in  a  strong  electric  arc,  but  no 
satisfactory  method  of  reckoning  the  scale  of  this  heat  has 
been  devised.  The  probabilities  are  to  the  effect  that  the 
heat  is  to  be  counted  by  the  tens  of  thousands  of  degrees 
Fahrenheit,  and  it  may  amount  to  hundreds  of  thousands; 
it  has,  indeed,  been  reckoned  as  high  as  a  million  degrees. 
This  vast  discharge  is  not  due  to  any  kind  of  burning  action 
— i.  e.,  to  the  combustion  of  substances,  as  in  a  fire.  It 
must  be  produced  by  the  gradual  falling  in  of  the  mate- 
rials, due  to  the  gravitation  of  the  mass  toward  its  centre, 
each  particle  converting  its  energy  of  position  into  heat, 
as  does  the  meteorite  when  it  comes  into  the  air. 

It  is  well  to  close  this  very  imperfect  account  of  the 
learning  which  relates  to  the  sun  with  a  brief  tabular  state- 
ment showing  the  relative  masses  of  the  several  bodies  of 
the  solar  system.  It  should  be  understood  that  by  mass  is 
meant  not  the  bulk  of  the  object,  but  the  actual  amount 
of  matter  in  it  as  determined  by  the  gravitative  attraction 
which  it  exercises  on  other  celestial  bodies.  In  this  test 
the  sun  is  taken  as  the  measure,  and  its  mass  is  for  con- 
venience reckoned  at  1,000,000,000. 


THE  STELLAR  REALM.  77 

Table  of  Relative  Masses  of  Sun  and  Planets.* 

The  sun 1,000,000,000 

Mercury 200 

Venus 2,353 

Earth 3,060 

Mars 339 

Asteroids f 

Saturn 285,580 

Jupiter 954,305 

Uranus 44,250 

Neptune 51,600 

Combined  mass  of  the  four  inner  planets  5,952 

Combined  mass  of  all  the  planets 1,341,687 

It  thus  appears  that  the  mass  of  all  the  planets  is  about 
one  seven  hundredth  that  of  the  sun. 

Those  who  wish  to  make  a  close  study  of  celestial  geog- 
raphy will  do  well  to  procure  the  interesting  set  of  dia- 
grams prepared  by  the  late  James  Freeman  Clarke,  in 
which  transparencies  placed  in  a  convenient  lantern  show 
the  grouping  of  the  important  stars  in  each  constellation. 
The  advantage  of  this  arrangement  is  that  the  little  maps 
can  be  consulted  at  night  and  in  the  open  air  in  a  very 
convenient  manner.  After  the  student  has  learned  the 
position  of  a  dozen  of  the  constellations  visible  in  the 
northern  hemisphere,  he  can  rapidly  advance  his  knowl- 
edge in  the  admirable  method  invented  by  Dr.  Clarke. 

Having  learned  the  constellations,  the  student  may 
well  proceed  to  find  the  several  planets,  and  to  trace  them 
in  their  apparent  path  across  the  fixed  stars.  It  will  be 
well  for  him  here  to  gain  if  he  can  the  conception  that 
their  apparent  movement  is  compounded  of  their  motion 
around  the  sun  and  that  of  our  own  sphere;  that  it  would 
be  very  different  if  our  earth  stood  still  in  the  heavens. 
At  this  stage  he  may  well  begin  to  take  in  mind  the  evi- 
dence which  the  planetary  motion  supplies  that  the  earth 

*  See  Newcomb's  Popular  Astronomy,  p.  234.  Harper  Brothers, 
New  York. 


78  OUTLINES  OF  THE  EARTH'S  HISTORY. 

really  moves  round  the  sun,  and  not  the  sun  and  planets 
round  the  earth.  This-  discovery  was  one  of  the  great 
feats  of  the  human  mind;  it  baffled  the  wits  of  the  best 
men  for  thousands  of  years.  Therefore  the  inquirer  who 
works  over  the  evidence  is  treading  one  of  the  famous  paths 
by  which  his  race  climbed  the  steeps  of  science. 

The  student  must  not  expect  to  find  the  evidence  that 
the  sun  is  the  centre  of  the  solar  system  very  easy  to  in- 
terpret; and  yet  any  youth  of  moderate  curiosity,  and  that 
interest  in  the  world  about  him  which  is  the  foundation  of 
scientific  insight,  can  see  through  the  matter.  He  will  best 
begin  his  inquiries  by  getting  a  clear  notion  of  the  fact 
that  the  moon  goes  round  the  earth.  This  is  the  simplest 
case  of  movements  of  this  nature  which  he  can  see  in  the 
solar  system.  Noting  that  the  moon  occupies  a  different 
place  at  a  given  hour  in  the  twenty-four,  but  is  evidently 
at  all  times  at  about  the  same  distance  from  the  earth,  he 
readily  perceives  that  it  circles  about  our  sphere.  This  the 
people  knew  of  old,  but  they  made  of  it  an  evidence  that 
the  sun  also  went  around  our  sphere.  Here,  then,  is  the 
critical  point.  Why  does  the  sun  not  behave  in  the  same 
manner  as  the  moon?  At  this  stage  of  his  inquiry  the 
student  best  notes  what  takes  place  in  the  motions  of  the 
planets  between  the  earth  and  the  sun.  He  observes  that 
those  so-called  inferior  planets  Mercury  and  Venus  are 
never  very  far  away  from  the  central  body;  that  they  appear 
to  rise  up  from  it,  and  then  to  go  back  to  it,  and  that  they 
have  phases  like  the  moon.  Now  and  then  Venus  may  be 
observed  as  a  black  spot  crossing  the  disk  of  the  sun.  A 
little  consideration  will  show  that  on  the  theory  that 
bodies  revolve  round  each  other  in  the  solar  system  these 
movements  of  the  inner  planets  can  only  be  explained  on 
the  supposition  that  they  at  least  travel  around  the  great 
central  fire.  Now,  taking  up  the  outer  planets,  we  ob- 
serve that  they  occasionally  appear  very  bright,  and  that 
they  are  then  at  a  place  in  the  heavens  where  we  see  that 
they  are  far  from  the  solar  centre.     Gradually  they  move 


THE  STELLAR  REALM.  79 

down  toward  the  sunset  and  disappear  from  view.  Here, 
too,  the  movement,  though  less  clearly  so,  is  best  recon- 
cilable with  the  idea  that  these  bodies  travel  in  orbits,  such 
as  those  which  are  traversed  by  the  inner  planets.  The 
wonder  is  that  with  these  simple  facts  before  them,  and 
with  ample  time  to  think  the  matter  over,  the  early  astrono- 
mers did  not  learn  the  great  truth  about  the  solar  system — 
namely,  that  the  sun  is  the  centre  about  which  the  planets 
circled.  Their  difficulty  lay  mainly  in  the  fact  that  they 
did  not  conceive  the  earth  as  a  sphere,  and  even  after  they 
attained  that  conception  they  believed  that  our  globe  was 
vastly  larger  than  the  planets,  or  even  than  the  sun.  This 
misconception  kept  even  the  thoughtful  Greeks,  who  knew 
that  the  earth  was  spherical  in  form,  from  a  clear  notion 
as  to  the  structure  of  our  system.  It  was  not,  indeed,  until 
mathematical  astronomy  attained  a  considerable  advance, 
and  men  began  to  measure  the  distances  in  the  solar  sys- 
tem, and  until  the  Newtonian  theory  of  gravitation  was 
developed,  that  the  planetary  orbits  and  the  relation  of  the 
various  bodies  in  the  solar  system  to  each  other  could  be 
perfectly  discerned. 

Care  has  been  taken  in  the  above  statements  to  give 
the  student  indices  which  may  assist  him  in  working  out 
for  himself  the  evidence  which  may  properly  lead  a  person, 
even  without  mathematical  considerations  of  a  formal 
kind,  to.  construct  a  theory  as  to  the  relation  of  the  planets 
to  the  sun.  It  is  not  likely  that  he  can  go  through  all  the 
steps  of  this  argument  at  once,  but  it  will  be  most  useful 
to  him  to  ponder  upon  the  problem,  and  gradually  win  h's 
way  to  a  full  understanding  of  it.  With  that  purpose  in 
mind,  he  should  avoid  reading  what  astronomers  have  to 
say  on  the  matter  until  he  is  satisfied  that  he  has  done  as 
much  as  he  can  with  the  matter  on  his  own  account.  He 
should,  however,  state  his  observations,  and  as  far  as  pos- 
sible draw  the  results  in  his  note-book  in  a  diagrammatic 
form.  He  should  endeavour  to  see  if  the  facts  are  recon- 
cilable with  any  other  supposition  than  that  the  earth  and 


80  OUTLINES  OF  THE  EARTH'S  HISTORY. 

the  other  planets  move  around  the  sun.  When  he  has  done 
his  task,  he  will  have  passed  over  one  of  the  most  difficult 
roads  which  his  predecessors  had  to  traverse  on  their  way 
to  an  understanding  of  the  heavens.  Even  if  he  fail  he 
will  have  helped  himself  to  some  large  understandings. 

The  student  will  find  it  useful  to  make  a  map  of  the 
heavens,  or  rather  make  several  representing  their  condi- 
tion at  different  times  in  the  year.  On  this  plot  he  should 
put  down  only  the  stars  whose  places  and  names  he  has 
learned,  but  he  should  plot  the  position  of  the  planets  at 
different  times.  In  this  way,  though  at  first  his  efforts 
will  be  very  awkward,  he  will  soon  come  to  know  the  gen- 
eral geography  of  the  heavens. 

Although  the  possession  or  at  least  the  use  of  a  small 
astronomical  telescope  is  a  great  advantage  to  a  student 
after  he  has  made  a  certain  advance  in  his  work,  such  an 
instrument  is  not  at  all  necessary,  or,  indeed,  desirable  at 
the  outset  of  his  studies.  An  ordinary  opera-glass,  how- 
ever, will  help  him  in  picking  out  the  stars  in  the  constella- 
tions, in  identifying  the  planets,  and  in  getting  a  better 
idea  as  to  the  form  of  the  moon's  surface — a  matter  which 
will  be  treated  in  this  work  in  connection  with  the  struc- 
ture of  the  earth. 


CHAPTER  ly. 

THE    EARTH. 

In  beginning  the  study  of  the  earth  it  is  important  that 
the  student  should  at  once  form  the  habit  of  keeping  in 
mind  the  spherical  form  of  the  planet.  Many  persons, 
while  they  may  blindly  accept  the  fact  that  the  earth  is 
a  sphere,  do  not  think  of  it  as  having  that  form.  Perhaps 
the  simplest  way  of  securing  the  correct  image  of  the  shape 
is  to  imagine  how  the  earth  would  appear  as  seen  from  the 
moon.  In  its  full  condition  the  moon  is  apt  to  appear  as 
a  disk.  When  it  is  new,  and  also  when  in  its  waning  stages 
it  is  visible  in  the  daytime,  the  spherical  form  is  very 
apparent.  Imagining  himself  on  the  surface  of  the  moon, 
the  student  can  well  perceive  how  the  earth  would  appear 
as  a  vast  body  in  the  heavens;  its  eight  thousand  miles  of 
diameter,  about  four  times  that  of  the  satellite,  would  give 
an  area  sixteen  times  the  size  which  the  moon  presents 
to  us.  On  this  scale  the  continents  and  oceans  would 
appear  very  much  more  plain  than  do  the  relatively  slight 
irregularities  on  the  lunar  surface. 

With  the  terrestrial  globe  in  hand,  the  student  can 
readily  construct  an  image  which  will  represent,  at  least  in 
outline,  the  appearance  which  the  sphere  he  inhabits  would 
present  when  seen  from  a  distance  of  about  a  quarter  of  a 
million  miles  away.  The  continent  of  Europe-Asia  would 
of  itself  appear  larger  than  all  the  lunar  surface  which  is 
visible  to  us.  Every  continent  and  all  the  greater  islands 
would  be  clearly  indicated.     The  snow  covering  which  in 

81 


82  OUTLINES  OF  THE  EARTH'S  HISTORY. 

the  winter  of  the  northern  hemisphere  wraps  so  much  of 
the  land  would  be  seen  to  come  and  go  in  the  changes  of 
the  seasons;  even  the  permanent  ice  about  either  pole, 
and  the  greater  regions  of  glaciers,  such  as  those  of  the 
Alps  and  the  Himalayas,  would  appear  as  brilliant  patches 
of  white  amid  fields  of  darker  hue.  Even  the  changes  in 
the  aspect  of  the  vegetation  w^hich  at  one  season  clothes 
the  wide  land  with  a  green  mantle,  and  at  another  as- 
sumes the  dun  hue  of  winter,  would  be,  to  the  unaided  eye, 
very  distinct.  It  is  probable  that  all  the  greater  rivers 
would  be  traceable  as  lines  of  light  across  the  relatively 
dark  surface  of  the  continents.  By  such  exercises  of  the 
constructive  imagination — indeed,  in  no  other  way — the 
student  can  acquire  the  habit  of  considering  the  earth 
as  a  vast  whole.  From  time  to  time  as  he  studies  the  earth 
from  near  by  he  should  endeavour  to  assemble  the  phe- 
nomena in  the  general  way  which  we  have  indicated. 

The  reader  has  doubtless  already  learned  that  the  earth 
is  a  slightly  flattened  sphere,  having  an  average  diameter 
of  about  eight  thousand  miles,  the  average  section  at  the 
equator  being  about  twenty-six  miles  greater  than  that 
from  pole  to  pole.  In  a  body  of  such  large  proportions  this 
difference  in  measurement  appears  not  important;  it  is, 
however,  most,  significant,  for  it  throws  light  upon  the  his- 
tory of  the  earth's  mass.  Computation  shows  that  the 
measure  of  flattening  at  the  poles  is  just  what  w^ould  occur 
if  the  earth  were  or  had  been  at  the  time  when  it  assumed 
its  present  form  in  a  fluid  condition.  We  readily  conceive 
that  a  soft  body  revolving  in  space,  while  all  its  particles 
by  gravitation  tended  to  the  centre,  would  in  turning 
around,  as  our  earth  does  upon  its  axis,  tend  to  bulge  out 
in  those  parts  which  were  remote  from  the  line  upon  which 
the  turning  took  place.  Thus  the  flattening  of  our  sphere 
at  the  poles  corroborates  the  opinion  that  its  mass  was  once 
molten — in  a  word,  that  its  ancient  history  was  such  as  the 
nebular  theory  suggests. 

Although  we  have  for  convenience  termed  the  earth 


THE  EARTH.  83 

a  flattened  spheroid,  it  is  only  such  in  a  very  general  sense. 
It  has  an  infinite  number  of  minor  irregularities  which  it 
is  the  province  of  the  geographer  to  trace  and  that  of  the 
geologist  to  account  for.  In  the  first  place,  its  surface  is 
occupied  by  a  great  array  of  ridges  and  hollows.  The 
larger  of  these,  the  oceans  and  continents,  first  deserve  our 
attention.  The  difference  in  altitude  of  the  earth's  sur- 
face from  the  height  of  the  continents  to  the  deepest  part 
of  the  sea  is  probably  between  ten  and  eleven  miles,  thus 
amounting  to  about  two  fifths  of  the  polar  flattening  be- 
fore noted.  The  average  difference  between  the  ocean  floor 
and  the  summits  of  the  neighbouring  continents  is  prob- 
ably rather  less  than  four  miles.  It  happens,  most  for- 
tunately for  the  history  of  the  earth,  that  the  water  upon 
its  surface  fills  its  great  concavities  on  the  average  to  about 
four  fifths  of  their  total  depth,  leaving  only  about  one 
fifth  of  the  relief  projecting  above  the  ocean  level.  We 
have  termed  this  arrangement  fortunate,  for  it  insures  that 
rainfall  visits  almost  all  the  land  areas,  and  thereby  makes 
those  realms  fit  for  the  uses  of  life.  If  the  ocean  had  only 
half  its  existing  area,  the  lands  would  be  so  wide  that  only 
their  fringes  would  be  fertile.  If  it  were  one  fifth  greater 
than  it  is,  the  dry  areas  would  be  reduced  to  a  few  scattered 
islands. 

From  all  points  of  view  the  most  important  feature  of 
the  earth's  surface  arises  from  its  division  into  land  and 
water  areas,  and  this  for  the  reason  that  the  physical  and 
vital  work  of  our  sphere  is  inevitably  determined  by  this 
distribution.  The  shape  of  the  seas  and  lands  is  fixed  by 
the  positions  at  which  the  upper  level  of  the  great  water 
comes  against  the  ridges  which  fret  the  earth's  surface. 
These  elevations  are  so  disposed  that  about  two  thirds 
of  the  hard  mass  is  at  the  present  time  covered  with  water, 
and  only  one  third  exposed  to  the  atmosphere.  This  pro- 
portion is  inconstant.  Owing  to  the  endless  up-and-down 
goings  of  the  earth's  surface,  the  place  of  the  shore  lines 
varies  from  year  to  year,  and  in  the  geological  ages  great 


84  OUTLINES  OF  THE  Ex\RTH'S  HISTORY. 

revolutions  in  the  forms  and  relative  area  of  water  and 
land  are  brought  about. 

Noting  the  greater  divisions  of  land  and  water  as  they 
are  shown  on  a  globe,  we  readily  perceive  that  those  parts 
of  the  continental  ridges  which  rise  above  the  sea  level 
are  mainly  accumulated  in  the  northern  hemisphere — in 
fact,  far  more  than  half  the  dry  realm  is  in  that  part  of 
the  world.  We  furthermore  perceive  that  all  the  conti- 
nents more  or  less  distinctly  point  to  the  southward;  they 
are,  in  a  word,  triangles,  with  their  bases  to  the  northward, 
and  their  apices,  usually  rather  acute,  directed  to  the  south- 
ward. This  form  is  very  well  indicated  in  three  of  the 
great  lands.  North  and  South  America  and  Africa;  it  is 
more  indistinctly  shown  in  Asia  and  in  Australia.  As  yet 
we  do  not  clearly  understand  the  reason  w^hy  the  continents 
are  triangular,  why  they  point  toward  the  south  pole,  or 
w^hy  they  are  mainly  accumulated  in  the  northern  hemi- 
sphere. As  stated  in  the  chapter  on  astronomy,  some  trace 
of  the  triangular  form  appears  in  the  land  masses  of  the 
planet  Mars.  There,  too,  these  triangles  appear  to  point 
toward  one  pole. 

Besides  the  greater  lands,  the  seas  are  fretted  by  a  host 
of  smaller  dry  areas,  termed  islands.  These,  as  inquiry 
has  shown,  are  of  two  very  diverse  natures.  Near  the  con- 
tinents, practically  never  more  than  a  thousand  miles  from 
their  shores,  we  find  isles,  often  of  great  size,  such  as  Mada- 
gascar, which  in  their  structure  are  essentially  like  the 
continents — that  is,  they  are  built  in  part  or  in  whole  of 
nonvolcanic  rocks,  sandstones,  limestones,  etc.  In  most 
cases  these  islands,  to  which  we  may  apply  the  term  con- 
tinental, have  at  some  time  been  connected  with  the  neigh- 
bouring mainland,  and  afterward  separated  from  it  by  a 
depression  of  the  surface  which  permitted  the  sea  to  flow 
over  the  lowlands.  Geologists  have  traced  many  cases 
where  in  the  past  elevations  which  are  now  parts  of  a  con- 
tinent were  once  islands  next  its  shore.  In  the  deeper  seas 
far  removed  from  the  margins  of  the  continents  the  islands 


THE  EARTH.  85 

are  made  up  of  volcanic  ejections  of  lava,  pumice,  and  dust, 
which  has  been  thrown  up  from  craters  and  fallen  around 
their  margin  or  are  formed  of  coral  and  other  organic 
remains. 

Next  after  this  general  statement  as  to  the  division  of 
sea  and  land  we  should  note  the  peculiarities  which  the 
earth^s  surface  exhibits  where  it  is  bathed  by  the  air,  and 
where  it  is  covered  by  the  water.  Beginning  with  the 
best-known  region,  that  of  the  dry  land,  we  observe  that 
the  surface  is  normally  made  up  of  continuous  slopes  of 
varying  declivity,  which  lead  down  from  the  high  points 
to  the  sea.  Here  and  there,  though  rarely,  these  slopes 
centre  in  a  basin  which  is  occupied  by  a  lake  or  a  dead  sea. 
On  the  deeper  ocean  floors,  so  far  as  we  may  judge  with 
the  defective  information  which  the  plumb  line  gives  us, 
there  is  no  such  continuity  in  the  downward  sloping  of 
the  surface,  the  area  being  cast  into  numerous  basins,  each 
of  great  extent. 

When  we  examine  in  some  detail  the  shape  of  the  land 
surface,  we  readily  perceive  that  the  continuous  down 
slopes  are  due  to  the  cutting  action  of  rivers.  In  the  basin 
of  a  stream  the  waters  act  to  wear  away  the  original  heights, 
filling  them  into  the  hollows,  until  the  whole  area  has  a 
continuous  down  grade  to  the  point  where  the  waters  dis- 
charge into  the  ocean  or  perhaps  into  a  lake.  On  the  bot- 
tom of  the  sea,  except  near  the  margin  of  the  continent, 
where  the  floor  may  in  recent  geological  times  have  been 
elevated  into  the  air,  and  thus  exposed  to  river  action,  there 
is  no  such  agent  working  to  produce  continuous  down 
grades. 

Looking  upon  a  map  of  a  continent  which  shows  the 
differences  in  altitude  of  the  land,  we  readily  perceive  that 
the  area  is  rather  clearly  divided  into  two  kinds  of  surface, 
mountains  and  plains,  each  kind  being  sharply  distin- 
guished from  the  other  by  many  important  peculiarities. 
Mountains  are  characteristically  made  up  of  distinct,  more 
or  less  parallel  ridges  and  valleys,  which  are  grouped  in 


80  OUTLINES  OF  THE  EARTH'S  HISTORY. 

very  elongated  belts,  which,  in  the  case  of  the  American 
Cordilleras,  extend  from  the  Arctic  to  the  Antarctic  Cir- 
cle. Only  in  rare  instances  do  we  find  mountains  occiip)^- 
ing  an  area  which  is  not  very  distinctly  elongated,  and  in 
such  cases  the  elevations  are  usually  of  no  great  height. 
Plains,  on  the  other  hand,  commonly  occupy  the  larger 
part  of  the  continent,  and  are  distributed  around  the  flanks 
of  the  mountain  systems.  There  is  no  rule  as  to  their 
shape;  they  normally  grade  away  from  the  bases  of  the 
mountains  toward  the  sea,  and  are  often  prolonged  below 
the  level  of  the  water  for  a  considerable  distance  beyond 
the  shore,  forming  what  is  commonly  known  as  the  conti- 
nental shelf  or  belt  of  shallows  along  the  coast  line.  We 
will  now  consider  some  details  concerning  the  form  and 
structure  of  mountains. 

In  almost  any  mountain  region  a  glance  over  the  sur- 
face of  the  country  will  give  the  reader  a  clew  to  the  prin- 
cipal factor  which  has  determined  the  existence  of  these 
elevations.  Wherever  the  bed  rocks  are  revealed  he  will 
recognise  the  fact  that  they  have  been  much  disturbed. 
Almost  everywhere  the  strata  are  turned  at  high  angles; 
often  their  slopes  are  steeper  than  those  of  house  roofs, 
and  not  infrequently  they  stand  in  attitudes  where  they 
appear  vertical.  Under  the  surface  of  plains  bedded  rocks 
generally  retain  the  nearly  horizontal  position  in  which 
all  such  deposits  are  most  likely  to  be  found.  If  the  ob- 
server will  attentively  study  the  details  of  position  of  these 
tilted  rocks  of  mountainous  districts,  he  will  in  most  cases 
be  able  to  perceive  that  the  beds  have  been  flexed  or  folded 
in  the  manner  indicated  by  the  diagram.  Sometimes, 
though  rarely,  the  tops  of  these  foldings  or  arches  have 
been  preserved,  so  that  the  nature  of  the  movement  can 
be  clearly  discerned.  More  commonly  the-  upper  parts  of 
the  upward-arching  strata  have  been  cut  off  by  the  action 
of  the  decay-bringing  forces — frost,  flowing  water,  or  creep- 
ing ice  in  glaciers — so  that  only  the  downward  pointing 
folds  which  were  formed  in  the  mountain-making  are  well 


THE    EARTH. 


S7 


3  I 

OS      S 


00  :•« 


^      «• 


LiJ 


•'•^^*.'/  Mill  Mountain 


-Cold  Sulphur  Sprtngo 


Back  Crttk  Mountala 


'■Uttlt  Oalffnuture  Btver 


t-r* 


'y 


,  WMm  SprlHQt  fffete 


LIWi  north  Mtm 


mM 


Uttlt  />/««»  i 


S'^.  «/«»>*  Mountain 


iiin 

J* 


88  OUTLINES  OF  THE  EARTH'S  HISTORY. 

preserved,  and  these  are  almost  invariably  hidden  within 
the  earth. 

By  walking  across  any  considerable  mountain  chain, 
as,  for  instance,  that  of  the  Alleghanies,  it  is  generally  pos- 
sible to  trace  a  number  of  these  parallel  up-and-down  folds 
of  the  strata,  so  that  we  readily  perceive  that  the  original 
beds  had  been  packed  together  into  a  much  less  space 
than  they  at  first  occupied.  In  some  cases  we  could  prove 
that  the  shortening  of  the  line  has  amounted  to  a  hun- 
dred miles  or  more — in  other  words,  points  on  the  plain 
lands  on  either  side  of  the  mountain  range  which  now 
exists  may  have  been  brought  a  hundred  miles  or  so  nearer 
together  than  they  were  before  the  elevations  were  pro- 
duced. The  reader  can  make  for  himself  a  convenient  dia- 
gram showing  what  occurred  by  pressing  a  number  of 
leaves  of  this  book  so  that  the  sheets  of  paper  are  thrown 
into  ridges  and  furrows.  By  this  experiment  he  also  will 
see  that  the  easiest  way  to  account  for  such  foldings  as 
we  observe  in  mountains  is  by  the  supposition  that  some 
force  residing  in  the  earth  tends  to  shove  the  beds  into 
a  smaller  space  than  they  originally  occupied.  Not  only 
are  the  rocks  composing  the  mountains  much  folded,  but 
they  are  often  broken  through  after  the  manner  of  masonry 
which  has  been  subjected  to  earthquake  shocks,  or  of  ice 
which  has  been  strained  by  the  expansion  that  affects  it  as 
it  becomes  warmed  before  it  is  melted.  In  fact,  many  of 
our  small  lakes  in  New  England  and  in  other  countries  of  a 
long  winter  show  in  a  miniature  way  during  times  of  thaw- 
ing ice  folds  which  much  resemble  mountain  arches. 

At  first  geologists  were  disposed  to  attribute  all  the 
phenomena  of  mountain-folding  to  the  progressive  cooling 
of  the  earth.  Although  this  sphere  has  already  lost  a  large 
part  of  the  heat  with  which  it  was  in  the  beginning  en- 
dowed, it  is  still  very  hot  in  its  deeper  parts,  as  is  shown 
by  the  phenomena  of  volcanoes.  This  internal  heat,  which 
to  the  present  day  at  the  depth  of  a  hundred  miles  below 
the  surface  is  probably  greater  than  that  of  molten  iron. 


THE  EARTH.  89 

is  constantly  flowing  away  into  space;  probably  enough  of 
it  goes  away  on  the  average  each  day  to  melt  a  hundred 
cubic  miles  or  more  of  ice,  or,  in  more  scientific  phrase,  the 
amount  of  heat  rendered  latent  by  melting  that  volume 
of  frozen  water.  J.  E.  Meyer,  an  eminent  physicist,  esti- 
mated the  quantity  of  heat  so  escaping  each  day  of  the  year 
to  be  sufficient  to  melt  two  hundred  and  forty  cubic  miles 
of  ice.  The  effect  of  this  loss  of  heat  is  constantly  to  shrink 
the  volume  of  the  earth;  it  has,  indeed,  been  estimated 
that  the  sphere  on  this  account  contracts  on  the  average 
to  the  amount  of  some  inches  each  thousand  years.  For 
the  reason  that  almost  all  this  heat  goes  from  the  depths 
of  the  earth,  the  cool  outer  portion  losing  no  considerable 
part  of  it,  the  contraction  that  is  brought  about  affects 
the  interior  portions  of  the  sphere  alone.  The  inner  mass 
constantly  shrinking  as  it  loses  heat,  the  outer,  cold  part 
is  by  its  weight  forced  to  settle  down,  and  can  only  accom- 
plish this  result  by  wrinkling.  An  analogous  action  may 
be  seen  where  an  apple  or  a  potato  becomes  dried;  in  this 
case  the  hard  outer  rind  is  forced  to  wrinkle,  because, 
losing  no  water,  it  does  not  diminish  in  its  extent,  and 
can  only  accommodate  itself  to  the  interior  by  a  wrinkling 
process.  In  one  case  it  is  water  which  escapes,  in  the 
other  heat;  but  in  both  contraction  of  the  part  which 
suffers  the  loss  leads  to  the  folding  of  the  outside  of  the 
spheroid. 

Although  this  loss  of  heat  on  the  part  of  the  earth 
accounts  in  some  measure  for  the  development  of  moun- 
tains, it  is  not  of  itself  sufficient  to  explain  the  phenomena, 
and  this  for  the  reason  that  mountains  appear  in  no  case 
to  develop  on  the  floors  of  the  wide  sea.  The  average 
depth  of  the  ocean  is  only  fifteen  thousand  feet,  while  there 
are  hundreds,  if  not  thousands,  of  mountain  crests  which 
exceed  that  height  above  the  sea.  Therefore  if  mountains 
grew  on  the  sea  floor  as  they  do  upon  the  land,  there  should 
be  thousands  of  peaks  rising  above  the  plain  of  the  waters, 
while,  in  fact,  all  of  the  islands  except  those  near  the  shores 


90  OUTLINES  OF  THE  EARTH'S  HISTORY. 

of  continents  are  of  volcanic  origin — that  is,  are  lands  of 
totally  different  nature. 

Whenever  a  considerable  mountain  chain  is  formed,  al- 
though the  actual  folding  of  the  beds  is  limited  to  the  usu- 
ally narrow  field  occupied  by  these  disturbances,  the  ele- 
vation takes  place  over  a  wide  belt  of  country  on  one  or 
both  sides  of  the  range.  Thus  if  we  approach  the  Eocky 
Mountains  from  the  Mississippi  Valley,  we  begin  to  mount 
up  an  inclined  plane  from  the  time  we  pass  westward  from 
the  Mississippi  River.  The  beds  of  rock  as  well  as  the  sur- 
face rises  gradually  until  at  the  foot  of  the  mountain; 
though  the  rocks  are  still  without  foldings,  they  are  at  a 
height  of  four  or  five  thousand  feet  above  the  sea.  It 
seems  probable — indeed,  we  may  say  almost  certain — that 
when  the  crust  is  broken,  as  it  is  in  mountain-building, 
by  extensive  folds  and  faults,  the  matter  which  lies  a  few 
score  miles  below  the  crust  creeps  in  toward  those  frac- 
tures, and  so  lifts  up  the  country  on  which  they  lie.  When 
we  examine  the  forms  of  any  of  our  continents,  we  find 
that  these  elevated  portions  of  the  earth's  crust  appear 
to  be  made  up  of  mountains  and  the  table-lands  which 
fringe  those  elevations.  There  is  not,  as  some  of  our 
writers  suppose,  two  different  kinds  of  elevation  in  our 
great  lands — the  continents  and  the  mountains  which  they 
bear — but  one  process  of  elevation  by  which  the  foldings 
and  the  massive  uplifts  which  constitute  the  table-lands 
are  simultaneously  and  by  one  process  formed. 

Looking  upon  continents  as  the  result  of  mountain 
growth,  we  may  say  that  here  and  there  on  the  earth's  crust 
these  dislocations  have  occurred  in  such  association  and  of 
such  magnitude  that  great  areas  have  been  uplifted  above 
the  plain  of  the  sea.  In  general,  we  find  these  groups  of 
elevations  so  arranged  that  they  produce  the  triangular 
form  which  is  characteristic  of  the  great  lands.  It  will 
be  observed,  for  instance,  that  the  form  of  ISTorth  America 
is  in  general  determined  by  the  position  of  the  Appalachian 
and  Cordilleran  systems  on  its  eastern  and  western  mar- 


tb 


THE  EARTH.  91 

gins,  though  there  are  a  number  of  smaller  chains,  such 
as  the  Laurentians  in  Canada  and  the  ice-covered  moun- 
tains of  Greenland,  which  have  a  measure  of  influence  in 
fixing  its  shore  lines. 

The  history  of  plains,  as  well  as  that  of  mountains, 
will  have  further  light  thrown  upon  it  when  in  the  next 
chapter  we  come  to  consider  the  effect  of  rain  water  on  the 
land.  We  may  here  note  the  fact  that  the  level  surfaces 
which  are  above  the  seashores  are  divisible  into  two  main 
groups — those  which  have  been  recently  lifted  above  the 
sea  level,  composed  of  materials  laid  down  in  the  shal- 
lows next  the  shore,  and  which  have  not  yet  shared  in 
mountain-building  disturbances,  and  those  which  have 
been  slightly  tilted  in  the  manner  before  indicated  in  the 
case  of  the  plains  which  border  the  Rocky  Mountains  on 
the  east.  The  great  southern  plain  of  eastern  and  southern 
United  States,  extending  from  near  New  York  to  Mexico, 
is  a  good  specimen  of  the  level  lands  common  on  all  the 
continents  which  have  recently  emerged  from  the  sea.  The 
table-lands  on  either  side  of  the  Mississippi  Valley,  slop- 
ing from  the  AUeghanies  and  the  Cordilleras,  represent 
the  more  ancient  type  of  plain  which  has  already  shared 
in  the  elevation  which  mountain-building  brings  about. 
In  rarer  cases  plains  of  small  area  are  formed  where  moun- 
tains formerly  existed  by  the  complete  moving  down  of 
the  original  ridges. 

There  is  a  common  opinion  that  the  continents  are 
liable  in  the  course  of  the  geologic  ages  to  very  great 
changes  of  position;  that  what  is  now  sea  may  give  place 
to  new  great  lands,  and  that  those  already  existing  may 
utterly  disappear.  This  opinion  was  indeed  generally  held 
by  geologists  not  more  than  thirty  years  ago.  Further 
study  of  the  problem  has  shown  us  that  while  parts  of 
each  continent  may  at  any  time  be  depressed  beneath  the 
sea,  the  whole  of  its  surface  rarely  if  ever  goes  below  the 
water  level.  Thus,  in  the  case  of  North  America,  we  can 
readily  note  very  great  changes  in  its  form  since  the  land 


92  OUTLINES  OF  THE  EARTH'S  HISTORY. 

began  to  rise  above  the  water.  But  always,  from  that 
ancient  day  to  our  own,  some  portion  of  the  area  has  been 
above  the  level  of  the  sea,  thus  providing  an  ark  of  refuge 
for  the  land  life  when  it  was  disturbed  by  inundations. 
The  strongest  evidence  in  favour  of  the  opinion  that  the 
existing  continents  have  endured  for  many  million  years 
is  found  in  the  fact  that  each  of  the  great  lands  preserves 
many  distinct  groups  of  animals  and  plants  which  have 
descended  from  ancient  forms  dwelling  upon  the  same 
territory.  If  at  any  time  the  relatively  small  continent  of 
Australia  had  gone  beneath  the  sea,  all  of  the  curious 
pouched  animals  akin  to  the  opossum  and  kangaroo  which 
abound  in  that  country — creatures  belonging  in  the  an- 
cient life  of  the  world — would  have  been  overwhelmed. 

We  have  already  noted  the  fact  that  the  uplifting  of 
mountains  and  of  the  table-lands  about  them,  which  ap- 
pears to  have  been  the  basis  of  continental  growth,  has 
been  due  to  strains  in  the  rocks  sufficiently  strong  to  dis- 
turb the  beds.  At  each  stage  of  the  mountain-building 
movement  these  compressive  strains  have  had  to  contend 
with  the  very  great  weight  of  the  rocks  which  they  had  to 
move.  These  lands  are  not  to  be  regarded  as  firm  set  or 
rigid  arches,  but  as  highly  elastic  structures,  the  shapes 
of  which  may  be  determined  by  any  actions  which  put  on 
or  take  off  burden.  We  see  a  proof  of  Ih's  fact  from  numer- 
ous observations  which  geologists  are  now  engaged  in  mak- 
ing. Thus  during  the  last  ice  epoch,  when  almost  all  the 
northern  part  of  this  continent,  as  well  as  the  northern 
part  of  Europe,  was  covered  by  an  ice  sheet  several  thou- 
sand feet  thick,  the  lands  sank  down  under  their  load,  and 
to  an  extent  roughly  proportional  to  the  depth  of  the  icy 
covering.  While  the  northern  regions  were  thus  tilted 
down  by  the  weight  which  was  upon  them,  the  southern 
section  of  this  land,  the  region  about  the  Gulf  of  Mexico, 
was  elevated  much  above  its  present  level;  it  seems  likely, 
indeed,  that  the  peninsula  of  Florida  rose  to  the  height  of 
several  hundred  feet  above  its  present  shore  line.     After 


THE  EARTH.  93 

the  ice  passed  away  the  movements  were  reversed,  the 
northern  region  rising  and  the  southern  sinking  down. 
These  movements  are  attested  by  the  position  of  the  old 
shore  lines  formed  during  the  later  stages  of  the  Glacial 
epoch.  Thus  around  Lake  Ontario,  as  well  as  the  other 
Great  Lakes,  the  beaches  which  mark  the  higher  positions 
of  those  inland  seas  during  the  closing  stages  of  the  ice 
time,  and  which,  of  course,  were  when  formed  horizontal, 
now  rise  to  the  northward  at  the  rate  of  from  two  to  five 
feet  for  each  mile  of  distance.  Recent  studies  by  Mr.  G. 
K.  Gilbert  show  that  this  movement  is  still  in  progress. 

Other  evidence  going  to  show  the  extent  to  which  the 
movements  of  the  earth's  crust  are  affected  by  the  weight 
of  materials  are  found  in  the  fact  that  wherever  along  the 
shores  thick  deposits  of  sediments  are  accumulated  the 
tendency  of  the  region  where  they  lie  is  gradually  to  sink 
downward,  so  that  strata  having  an  aggregate  thickness 
of  ten  thousand  feet  or  more  may  be  accumulated  in  a  sea 
which  was  always  shallow.  The  ocean  floor,  in  general,  is 
the  part  of  the  earth's  surface  where  strata  are  constantly 
being  laid  down.  In  the  great  reservoir  of  the  waters  the 
debris  washed  from  the  land,  the  dust  from  volcanoes,  and 
that  from  the  stellar  spaces,  along  with  the  vast  accumula- 
tion of  organic  remains,  almost  everywhere  lead  to  the 
steadfast  accumulation  of  sedimentary  deposits.  On  the 
other  hand,  the  realms  of  the  surface  above  the  ocean  level 
are  constantly  being  worn  away  by  the  action  of  the  rivers 
and  glaciers,  of  the  waves  which  beat  against  the  shores, 
and  of  the  winds  which  blow  over  desert  regions.  The 
result  is  that  the  lands  are  wearing  down  at  the  geologic- 
ally rapid  average  rate  of  somewhere  about  one  foot  in  five 
thousand  years.  All  this  heavy  matter  goes  to  the  sea 
bottoms.  Probably  to  this  cause  we  owe  in  part  the  fact 
that  in  the  wrinklings  of  the  crust  due  to  the  contraction 
of  the  interior  the  lands  exhibit  a  prevailing  tendency  to 
uprise,  while  the  ocean  floors  sink  down.  In  this  way  the 
continents  are  maintained  above  the  level  of  the  sea  de- 


94 


OUTLINES  OF  THE  EARTH'S  HISTORY. 


spite  the  powerful  forces  which  are  constantly  wearing 
their  substance  away,  while  the  seas  remain  deep,  although 
they  are  continually  being  burdened  with  imported  ma- 
terials. 


Fig.  8. — Diagram  showing  the  effect  of  the  position  of  the  fulcrum 
point  in  the  movement  of  the  land  masses.  In  diagrams  I  and 
II,  the  lines  a  b  represent  the  land  before  the  movement,  and  a'  b' 
its  position  after  the  movement ;  s,  5,  the  position  of  the  shore 
line  ;  p,  p,  the  pivotal  points ;  I,  s,  the  sea  line.  In  diagram  III, 
the  curved  line  designates  a  shore ;  the  line  a  b,  connecting  the 
pivotal  points  p,p,  is  partly  under  the  land  and  partly  under 
the  sea. 


It  is  easy  to  see  that  if  the  sea  floors  tend  to  sink  down- 
ward, while  the  continental  lands  uprise,  the  movements 
which  take  place  may  be  compared  with  those  which  occur 
in  a  lever  about  a  fulcrum  point.    In  this  case  the  sea  end 


THE  EARTH.  95 

of  the  bar  is  descending  and  the  land  end  ascending.  Now, 
it  is  evident  that  the  fulcrum  point  may  fall  to  the  seaward 
or  to  the  landward  of  the  shore;  only  by  chance  and  here 
and  there  would  it  lie  exactly  at  the  coast  line.  By  refer- 
ence to  the  diagram  (Fig.  8),  it  will  be  seen  that,  while 
the  point  of  rotation  is  just  at  the  shore,  a  considerable 
movement  may  take  place  without  altering  the  position 
of  the  coast  line.  Where  the  point  of  no  movement  is 
inland  of  the  coast,  the  sea  will  gain  on  the  continent; 
where,  however,  the  point  is  to  seaward,  beneath  the  water, 
the  land  will  gain  on  the  ocean.  In  this  way  we  can,  in 
part  at  least,  account  for  the  endless  changes  in  the  atti- 
tude of  the  land  along  the  coastal  belt  without  having  to 
suppose  that  the  continents  cease  to  rise' or  the  sea  floors 
to  sink  downward.  It  is  evident  that  the  bar  or  section 
of  the  rocks  from  the  interior  of  the  land  to  the  bottoms 
of  the  seas  is  not  rigid;  it  is  also  probable  that  the  matter 
in  the  depths  of  the  earth,  which  moves  with  the  motions 
of  this  bar,  would  change  the  position  of  the  fulcrum  point 
from  time  to  time.  Thus  it  may  well  come  about  that  our 
coast  lines  are  swaying  up  and  down  in  ceaseless  variation. 
In  very  recent  geological  times,  probably  since  the  be- 
ginning of  the  last  Glacial  period,  the  region  about  the 
Dismal  Swamp  in  Virginia  has  swayed  up  and  down 
through  four  alternating  movements  to  the  extent  of  from 
fifty  to  one  hundred  feet.  The  coast  of  New  Jersey  is 
now  sinking  at  the  rate  of  about  two  feet  in  a  hundred 
years.  The  coast  of  New  England,  though  recently  ele- 
vated to  the  extent  of  a  hundred  feet  or  more,  at  a  yet  later 
time  sank  down,  so  that  at  some  score  of  points  between 
New  York  and  Eastport,  Me.,  we  find  the  remains  of  for- 
ests with  the  roots  of  their  trees  still  standing  below  high- 
tide  mark  in  positions  where  the  trees  could  not  have 
grown.  Along  all  the  marine  coasts  of  the  world  which 
have  been  carefully  studied  from  this  point  of  view  there 
are  similar  evidences  of  slight  or  great  modern  changes 
in  the  level  of  the  lands.     At  some  points,  particularly 


96  OUTLINES  OF  THE  EARTH'S  HISTORY. 

on  the  coast  of  Alaska  and  along  the  coast  of  Peru,  these 
uplifts  of  the  land  have  amounted  to  a  thousand  feet  or 
more.  In  the  peninsular  district  of  Scandinavia  the  sway- 
ings,  sometimes  up  and  sometimes  down,  which  are  now 
going  on  have  considerably  changed  the  position  of  the 
shore  lines  since  the  beginning  of  the  historical  period. 

There  are  other  causes  which  serve  to  modify  the  shapes 
and  sizes  of  the  continents  which  may  best  be  considered 
in  the  sequel;  for  the  present  we  may  pass  from  this  sub- 
ject with  the  statement  that  our  great  lands  are  relatively 
permanent  features;  their  forms  change  from  age  to  age, 
but  they  have  remained  for  millions  of  years  habitable  to 
the  hosts  of  animals  and  plants  which  have  adapted  their 
life  to  the  conditions  which  these  fields  afford  them. 


CHAPTEE  V. 

THE   ATMOSPHERE. 

The  firm-set  portion  of  the  earth,  composed  of  mate- 
rials which  became  solid  when  the  heat  so  far  disappeared 
from  the  sphere  that  rocky  matter  could  pass  from  its  pre- 
vious fluid  condition  to  the  solid  or  frozen  state,  is  wrapped 
about  by  two  great  envelopes,  the  atmosphere  and  the 
waters.  Of  these  we  shall  first  consider  the  lighter  and 
more  universal  air;  in  taking  account  of  its  peculiarities 
we  shall  have  to  make  some  mention  of  the  water  with 
which  it  is  greatly  involved;  afterward  we  shall  consider 
the  structure  and  functions  of  that  fluid. 

Atmospheric  envelopes  appear  to  be  common  features 
about  the  celestial  spheres.  In  the  sun  there  is,  as  we  have 
noted,  a  very  deep  envelope  of  this  sort  which  is  in  part 
composed  of  the  elements  which  form  our  own  air;  but, 
owing  to  the  high  temperature  of  the  sphere,  these  are 
commingled  with  many  substances  which  in  our  earth — 
at  least  in  its  outer  parts — ^have  entered  in  the  solid  state. 
Some  of  the  planets,  so  far  as  we  can  discern  their  condi- 
tions, seem  also  to  have  gaseous  wraps;  this  is  certainly 
the  case  with  the  planet  Mars,  and  even  the  little  we  know 
of  the  other  like  spheres  justifies  the  supposition  that  Jupi- 
ter and  Saturn,  at  least,  have  a  like  constitution.  We  may 
regard  an  atmosphere,  in  a  word,  as  representing  a  normal 
and  long-continued  state  in  the  development  of  the  heav- 
enly orbs.  In  only  one  of  these  considerable  bodies  of  the 
solar  system,  the  moon,  do  we  find  tolerably  clear  evidence 
that  there  is  no  atmosphere. 

97 


98  OUTLINES  OF  THE  EARTH'S  HISTORY. 

The  atmosphere  of  the  earth  is  composed  mainly  of  very- 
volatile  elements,  known  as  nitrogen  and  argon.  This  is 
commingled  with  oxygen,  also  a  volatile  element.  Into 
this  mass  a  number  of  other  substances  enter  in  varying 
but  always  relatively  very  small  proportions.  Of  these  the 
most  considerable  are  watery  vapour  and  carbon  dioxide; 
the  former  of  these  rarely  amounts  to  one  per  cent  of  the 
weight  of  the  air,  considering  the  atmosphere  as  a  whole, 
and  the  latter  is  never  more  than  a  small  fraction  of  one 
per  cent  in  amount.  As  a  whole,  the  air  envelope  of  the 
earth  should  be  regarded  as  a  mass  of  nitrogen  and  argon, 
which  only  rarely,  under  the  influence  of  conditions  which 
exist  in  the  soil,  enters  into  combinations  with  other  ele- 
ments by  which  it  assumes  a  solid  form.  The  oxygen, 
though  a  permanent  element  in  the  atmosphere,  tends  con- 
stantly to  enter  into  combinations  which  fix  it  temporarily 
or  permanently  in  the  earth,  in  which  it  forms,  indeed,  in 
its  combined  state  about  one  half  the  weight  of  all  the 
mineral  substances  we  know.  The  carbon  dioxide,  or  car- 
bonic-acid gas,  as  it  is  commonly  termed,  is  a  most  im- 
portant substance,  as  it  affords  plants  all  that  part  of  their 
bodies  which  disappear  on  burning.  It  is  constantly  re- 
turned to  the  atmosphere  by  the  decay  of  organic  matter, 
as  well  as  by  volcanic  action. 

In  addition  to  the  above-noted  materials  composing 
the  air,  all  of  which  are  imperatively  necessary  to  the 
wonderful  work  accomplished  by  that  envelope,  we  find 
a  host  of  other  substances  which  are  accidentally,  variably, 
and  always  in  small  quantities  contained  in  this  realm. 
Thus  near  the  seashores,  and  indeed  for  a  considerable 
distance  into  the  continent,  we  find  the  air  contains  a 
certain  amount  of  salt  so  finely  divided  that  it  floats  in 
the  atmosphere.  So,  too,  we  find  the  air,  even  on  the 
mountain  tops  amid  eternal  snows,  charged  with  small 
particles  of  dust,  which,  though  not  evident  to  the  un- 
assisted eye,  become  at  once  visible  when  we  permit  a  slen- 
der ray  of  light  to  enter  a  dark  chamber. 


THE  ATMOSPHERE.  99 

It  is  commonly  asserted  that  the  atmosphere  does  not 
effectively  extend  above  the  height  of  forty-five  miles;  we 
know  that  it  is  densest  on  the  surface  of  the  earth,  the 
most  so  in  those  depressions  which  lie  below  the  level  of 
the  sea.  This  is  proved  to  us  by  the  weight  which  the  air 
imposes  upon  the  mercury  at  the  open  end  of  a  baro- 
metric tube.  If  we  could  deepen  these  cavities  to  the  ex- 
tent of  a  thousand  miles,  the  pressure  would  become  so 
great  that  if  the  pit  were  kept  free  from  the  heat  of  the 
earth  the  gaseous  materials  would  become  liquefied.  Up- 
ward from  the  earth's  surface  at  the  sea  level  the  atoms 
and  molecules  of  the  air  become  farther  apart  until,  at 
the  height  of  somewhere  between  forty  and  fifty  miles, 
the  quantity  of  them  contained  in  the  ether  is  so  small 
that  we  can  trace  little  effect  from  them  on  the  rays  of 
light  which  at  lower  levels  are  somewhat  bent  by  their 
action.  At  yet  higher  levels,  however,  meteors  appear 
to  inflame  by  friction  against  the  particles  of  air,  and  even 
at  the  height  of  eighty  miles  very  faint  clouds  have  at 
times  been  discerned,  which  are  possibly  composed  of  vol- 
canic dust  floating  in  the  very  rarefied  medium,  such  aa 
must  exist  at  this  great  elevation. 

The  air  not  only  exists  in  the  region  where  we  dis- 
tinctly recognise  it;  it  also  occupies  the  waters  and  the 
under  earth.  In  the  waters  it  occurs  as  a  mechanical  mix- 
ture which  is  brought  about  as  the  rain  forms  and  falls 
in  the  air,  as  the  streams  flow  to  the  sea,  and  as  the  waves 
roll  over  the  deep  and  beat  against  the  shores.  In  the 
realm  of  the  waters,  as  well  as  on  the  land,  the  air  is  neces- 
sary for  the  maintenance  of  all  animal  forms;  but  for  its 
presence  such  life  would  vanish  from  the  earth. 

Owing  to  certain  peculiarities  in  its  constitution,  the 
atmosphere  of  our  earth,  and  that  doubtless  of  myriad 
other  spheres,  serves  as  a  medium  of  communication  be- 
tween different  regions.  It  is,  as  we  know,  in  ceaseless 
motion  at  rates  which  may  vary  from  the  speed  in  the 
greatest  tempests,  which  may  move  at  the  rate  of  some- 


100  OUTLINES  OF  THE  EARTH'S  HISTORY. 

where  a  hundred  and  fifty  miles  an  hour,  to  the  very  slow 
movements  which  occur  in  caverns,  where  the  transfer  is 
sometimes  effected  at  an  almost  mocroscopic  rate  in  the 
space  of  a  day.  The  motion  of  the  atmosphere  is  brought 
about  by  the  action  of  heat  here  and  there,  and  in  a  trifling 
way,  by  the  heat  from  the  interior  of  the  earth  escaping 
through  hot  springs  or  volcanoes,  but  almost  altogether 
by  the  heat  of  the  sun.  If  we  can  imagine  the  earth  cut 
off  from  the  solar  radiation,  the  air  would  cease  to  move. 
We  often  note  how  the  variable  winds  fall  away  in  the 
nighttime.  Those  who  in  seeking  for  the  North  Pole  have 
spent  winters  in  the  long-continued  dark  of  that  region 
have  noted  that  the  winds  almost  cease  to  blow,  the  air 
being  disturbed  only  when  a  storm  originated  in  the  sunlit 
realm  forced  its  way  into  the  circumpolar  darkness. 

The  sun's  heat  does  not  directly  disturb  the  atmos- 
phere; if  we  could  take  the  solid  sphere  of  the  world  away, 
leaving  the  air,  the  rays  would  go  straight  through,  and 
there  would  be  no  winds  produced.  This  is  due  to  the 
fact  that  the  air  permits  the  direct  rays  of  heat,  such  as 
come  from  the  sun,  to  pass  through  it  with  very  slight 
resistance.  In  an  aerial  globe  such  as  we  have  imagined, 
the  rays  impinging  upon  its  surface  would  be  slightly 
thrown  out  of  their  path  as  they  are  in  passing  through  a 
lens,  but  they  would  journey  on  in  space  without  in  any 
considerable  measure  warming  the  mass.  Coming,  how- 
ever, upon  the  solid  earth,  the  heat  rays  warm  the  mate- 
rials on  which  they  are  arrested,  bringing  them  to  a  higher 
temperature  than  the  air.  Then  these  heated  materials 
radiate  the  energy  into  the  air;  it  happens,  however,  that 
this  radiant  heat  can  not  journey  back  into  space  as  easily 
as  it  came  in;  therefore  the  particles  of  air  next  the  sur- 
face acquire  a  relatively  high  temperature.  ■  Thus  a  ther- 
mometer next  the  ground  may  rise  to  over  a  hundred  de- 
grees Fahrenheit,  while  at  the  same  time  the  fleecy  clouds 
which  we  may  observe  floating  at  the  height  of  five  or  six 
miles  above  the  surface  are  composed  of  frozen  water. 


THE  AT]J[bsraEREr;;*v;;/ i\i  ;  ,101 


The  effect  of  the  heated  air  which  acquires  its  tem- 
perature by  radiation  from  the  earth's  surface  is  to  pro- 
duce the  winds.  This  it  brings  about  in  a  very  simple 
manner,  though  the  details  of  the  process  have  a  certain 
complication.  The  best  illustration  of  the  mode  in  which 
the  winds  are  produced  is  obtained  by  watching  what  takes 
place  about  an  ordinary  fire  at  the  bottom  of  a  chimney. 
As  soon  as  the  fire  is  lit,  we  observe  that  the  air  about  it, 
so  far  as  it  is  heated,  tends  upward,  drawing  the  smoke 
with  it.  If  the  air  in  the  chimney  be  cold,  it  may  not 
draw  well  at  first;  but  in  a  few  minutes  the  draught  is 
established,  or,  in  other  words,  the  heated  lower  air  breaks 
its  way  up  the  shaft,  gradually  pushing  the  cooler  matter 
out  at  the  top.  In  still  air  we  may  observe  the  column 
from  the  flue  extending  about  the  chimney-top,  sometimes 
to  the  height  of  a  hundred  feet  or  more  before  it  is  broken 
to  pieces.  It  is  well  here  to  note  the  fact  that  the  energy  of 
the  draught  in  a  chimney  is,  with  a  given  heat  of  fire  and 
amount  of  air  which  is  permitted  to  enter  the  shaft,  di- 
rectly proportionate  to  the  height;  thus  in  very  tall  flues, 
between  two  and  three  hundred  feet  high,  which  are  some- 
times constructed,  the  uprush  is  at  the  speed  of  a  gale. 

Whenever  the  air  next  the  surface  is  so  far  heated  that 
it  may  overcome  the  inertia  of  the  cooler  air  above,  it  forces 
its  way  up  through  it  in  the  general  manner  indicated  in 
the  chimney  flue.  When  such  a  place  of  uprush  is  estab- 
lished, the  hot  air  next  the  surface  flows  in  all  directions 
toward  the  shaft,  joining  the  expedition  to  the  heights 
of  the  atmosphere.  Owing  to  the  conditions  of  the  earth's 
surface,  which  we  shall  now  proceed  to  trace,  these  ascents 
of  heated  air  belong  in  two  distinct  classes — those  which 
move  upward  through  more  or  less  cylindrical  chimneys 
in  the  atmosphere,  shafts  which  are  impermanent,  which 
vary  in  diameter  from  a  few  feet  to  fifty  or  perhaps  a  hun- 
dred miles,  and  which  move  over  the  surface  of  the  earth; 
and  another  which  consists  of  a  broad,  beltlike  shaft  in 
the  equatorial  regions,  which  in  a  way  girdles  the  earth. 


102  .OUTLINES.  (:)P.T.HE  EARTH'S  HISTORY. 

remains  in  about  the  same  place,  continually  endures,  and 
lias  a  width  of  hundreds  of  miles.  Of  these  two  classes 
of  uprushes  we  shall  first  consider  the  greatest,  which 
occurs  in  the  central  portions  of  the  tropical  realm. 

Under  the  equator,  owing  to  the  fact  that  the  sun  for 
a  considerable  belt  of  land  and  sea  maintains  the  earth  at 
a  high  temperature,  there  is  a  general  updraught  which 
began  many  million  years  ago,  probably  before  the  origin 
of  life,  in  the  age  when  our  atmosphere  assumed  its  pres- 
ent conditions.  Into  this  region  the  cooler  air  from  the 
north  and  south  necessarily  flows,  in  part  pressed  in  by 
the  weight  of  the  cold  air  which  overlies  it,  but  aided  in 
its  motion  by  the  fact  that  the  particles  which  ascend 
leave  place  for  others  to  occupy.  Over  the  surfaces  of 
the  land  within  the  tropical  region  this  draught  toward 
what  we  may  term  the  equatorial  chimney  is  perturbed 
by  the  irregularities  of  the  surface  and  many  local  acci- 
dents. But  on  the  sea,  where  the  conditions  are  uniform, 
the  air  moving  toward  the  point  of  ascent  is  marked  in  the 
trade  winds,  which  blow  with  a  steadfast  sweep  down 
toward  the  equator.  Many  slight  actions,  such  as  the 
movement  of  the  hot  and  cold  currents  of  the  sea,  the 
local  air  movements  from  the  lands  or  from  detached 
islands,  somewhat  perturb  the  trade  winds,  but  they 
remain  among  the  most  permanent  features  in  this  change- 
able world.  It  is  doubtful  if  anything  on  this  sphere 
except  the  atoms  and  molecules  of  matter  have  varied  as 
little  as  the  trade  winds  in  the  centre  of  the  wide  ocean. 
So  steadfast  and  uniform  are  they  that  it  is  said  that  the 
helm  and  sails  of  a  ship  may  be  set  near  the  west  coast 
of  South  America  and  be  left  unchanged  for  a  voyage 
which  will  carry  the  navigator  in  their  belt  across  the 
width  of  the  Pacific. 

Eising  up  from  the  earth  in  the  tropical  belt,  the  air 
attains  the  height  of  several  thousand  feet;  it  then  begins 
to  curve  off  toward  the  north  and  south,  and  at  the  height 
of  somewhere  about  three  to  five  miles  above  the  surface 


THE  ATMOSPHEEE.  103 

is  again  moving  horizontally  toward  either  pole;  attaining 
a  distance  on  that  journey,  it  gradually  settles  down  to  the 
surface  of  the  earth,  and  ceases  to  move  toward  higher 
latitudes.  If  the  earth  did  not  revolve  upon  its  axis  the 
course  of  these  winds  along  the  surface  toward  the  equator, 
and  in  the  upper  air  back  toward  the  poles,  would  be  made 
in  what  we  may  call  a  square  manner — that  is,  the  parti- 
cles of  air  would  move  toward  the  point  where  they  begin 
to  rise  upw^ard  in  due  north  and  south  lines,  according 
as  they  came  from  the  southern  or  northern  hemisphere, 
and  the  upper  currents  or  counter  trades  would  retrace 
their  paths  also  parallel  with  the  meridians  or  longitude 
lines.  But  because  the  earth  revolves  from  west  to  east, 
the  course  of  the  trade  winds  is  oblique  to  the  equator, 
those  in  the  northern  hemisphere  blowing  from  northeast 
to  southwest,  those  in  the  southern  from  southeast  to 
northwest.  The  way  in  which  the  motion  of  the  earth 
affects  the  direction  of  these  currents  is  not  difficult  to 
understand.    It  is  as  follows: 

Let  us  conceive  a  particle  of  air  situated  immediately 
over  the  earth's  polar  axis.  Such  an  atom  would  by  the 
rotation  of  the  sphere  accomplish  no  motion  except,  in- 
deed, that  it  might  turn  round  on  its  own  centre.  It 
would  acquire  no  velocity  w^hatever  by  virtue  of  the  earth's 
movement.  Then  let  us  imagine  the  particle  moving  to- 
ward the  equator  with  the  speed  bi  an  ordinary  wind.  At 
every  step  of  its  journey  toward  low^er  latitudes  it  would 
come  into  regions  having  a  greater  movement  than  those 
which  it  had  just  left.  Owing  to  its  inertia,  it  would  thus 
tend  continually  to  lag  behind  the  particles  of  matter 
about  it.  It  would  thus  fall  off  to  the  w^estward,  and,  in 
place  of  moving  due  south,  would  in  the  northern  hemi- 
sphere drift  to  the  southwest,  and  in  the  southern  hemi- 
sphere toward  the  northwest.  A  good  illustration  of  th's 
action  may  be  obtained  from  an  ordinary  turn-table  such 
as  is  used  about  railway  stations  to  reverse  the  position 
of  a  locomotive.  If  the  observer  will  stand  in  the  centre 
8 


104  OUTLINES  OF  THE  EARTH'S  HISTORY. 

of  such  a  table  while  it  is  being  turned  round  he  will 
perceive  that  his  body  is  not  swayed  to  the  right  or  left. 
If  he  will  then  try  to  walk  toward  the  periphery  of  the 
rotating  disk,  he  will  readily  note  that  it  is  very  diffi- 
cult, if  not  impossible,  to  walk  along  the  radius  of  the 
circle;  he  naturally  falls  behind  in  the  movement,  so  that 
his  path  is  a  curved  line  exactly  such  as  is  followed  by  the 
winds  which  move  toward  the  equator  in  the  trades.  If 
now  he  rests  a  moment  on  the  periphery  of  the  table,  so  that 
his  body  acquires  the  velocity  of  the  disk  at  that  point,  and 
then  endeavours  to  walk  toward  the  centre,  he  will  find  that 
again  he  can  not  go  directly;  his  path  deviates  in  the  oppo- 
site direction — in  other  words,  the  body  continually  going 
to  a  place  having  a  less  rate  of  movement  by  virtue  of  the 
rotation  of  the  earth,  on  account  of  its  momentum  is  ever 
moving  faster  than  the  surface  over  which  it  passes.  This 
experiment  can  readily  be  tried  on  any  small  rotating  disk, 
such  as  a  potter's  wheel,  or  by  rolling  a  marble  or  a  shot 
from  the  centre  to  the  circumference  and  from  the  circum- 
ference to  the  centre.  A  little  reflection  will  show  the 
inquirer  how  these  illustrations  clearly  account  for  the 
oblique  though  opposite  sets  of  the  trade  winds  in  the 
upper  and  lower  parts  of  the  air. 

The  dominating  effect  of  the  tropical  heat  in  con- 
trolling the  movements  of  the  air  currents  extends,  on 
the  ocean  surface,  in  general  about  as  far  north  and  south 
as  the  parallels  of  forty  degrees,  considerably  exceeding 
the  limits  of  the  tropics,  those  lines  where  the  sun,  because 
of  the  inclination  of  the  earth's  axis,  at  some  time  of  the 
year  comes  just  overhead.  Between  these  belts  of  trade 
winds  there  is  a  strip  or  belt  under  the  region  where  the 
atmosphere  is  rising  from  the  earth,  in  which  the  winds 
are  irregular  and  have  little  energy.  This  region  of  the 
"  doldrums  "  or  frequent  calms  is  one  of  much  trouble  to 
sailing  ships  on  their  voyages  from  one  hemisphere  to  an- 
other. In  passing  through  it  their  sails  are  filled  only  by 
the  airs  of  local  storms,  or  winds  w^hich  make  their  way 


THE  ATMOSPHERE.  105 

into  that  part  of  the  sea  from  the  neighbouring  conti- 
nents. Beyond  the  trade-wind  belt,  toward  the  poles,  the 
movements  of  the  atmosphere  are  dependent  in  part  on  the 
counter  trades  which  descend  to  the  surface  of  the  earth 
in  latitudes  higher  than  that  in  which  the  surface  or  trade 
winds  flow.  Thus  along  our  Atlantic  coast,  and  even  in 
the  body  of  the  continent,  at  times  when  the  air  is  not 
controlled  by  some  local  storm,  the  counter  trade  blows 
with  considerable  regularity. 

The  effect  of  the  trade  and  counter-trade  movements 
of  the  air  on  the  distribution  of  temperature  over  the 
earth's  surface  is  momentous.  In  part  their  influence  is 
due  to  the  direct  heat-carrying  power  of  the  atmosphere; 
in  larger  measure  it  is  brought  about  by  the  movement  of 
the  ocean  waters  which  they  induce.  Atmospheric  air, 
when  deprived  of  the  water  which  it  ordinarily  contains, 
has  very  little  heat-containing  capacity.  Practically  nearly 
all  the  power  of  conveying  heat  which  it  possesses  is  due 
to  the  vapour  of  water  which  it  contains.  By  virtue  of 
this  moisture  the  winds  do  a  good  deal  to  transfer  heat 
from  the  tropical  or  superheated  portion  of  the  earth's 
surface  to  the  circumpolar  or  underheated  realms.  At 
first,  the  relatively  cool  air  which  journeys  toward  the 
equator  along  the  surface  of  the  sea  constantly  gains  in 
heat,  and  in  that  process  takes  up  more  and  more  water, 
for  precisely  the  same  reason  that  causes  anything  to  dry 
more  rapidly  in  air  which  has  been  warmed  next  a  fire. 
The  result  is  that  before  it  begins  to  ascend  in  the  tropical 
updraught,  being  much  moisture-laden,  the  atmosphere 
stores  a  good  deal  of  heat.  As  it  rises,  rarefies,  and  cools, 
the  moisture  descends  in  the  torrential  rains  which  ordi- 
narily fall  when  the  sun  is  nearly  vertical  in  the  tropical 
belt. 

Here  comes  in  a  very  interesting  principle  which  is 
of  importance  in  understanding  the  nature  of  great  storms, 
either  the  continuous  storm  of  the  tropics  or  the  local  and 
irregular  whirlings  which  occur  in  various  parts  of  the 


106  OUTLINES  OF  THE  EARTH'S  HISTORY. 

earth.  When  the  moisture-laden  air  starts  on  its  upward 
journey  from  the  earth  it  has,  by  virtue  of  the  watery 
vapour  which  it  contains,  a  store  of  energy  which  becomes 
applied  to  promoting  the  updraught.  As  it  rises,  the  moist- 
ure in  the  air  gathers  together  or  condenses,  and  in  so 
doing  parts  with  the  heat  which  caused  it  to  evaporate 
from  the  ocean  surface.  For  a  given  weight  of  water, 
the  amount  of  heat  required  to  effect  the  evaporation  is 
very  great;  this  we  may  roughly  judge  by  observing  what 
a  continuous  fire  is  required  to  send  a  pint  of  water  into 
the  state  of  steam.  This  energy,  when  it  is  released  by  the 
condensation  of  water  into  rain  or  snow,  becomes  again 
heat,  and  tends  somewhat,  as  does  the  fire  in  the  chimney, 
to  accelerate  the  upward  passage  of  the  air.  The  result  is 
that  the  water  which  ascends  in  the  equatorial  updraught 
becomes  what  we  may  term  fuel  to  promote  this  important 
element  in  the  earth's  aerial  circulation.  Trades  and  coun- 
ter trades  would  doubtless  exist  but  for  the  efficiency  of 
this  updraught,  which  is  caused  by  the  condensation  of 
watery  vapour,  but  the  movement  would  be  much  less  than 
it  is. 

Whikling  Storms. 

In  the  region  near  the  equator,  or  near  the  line  of  high- 
est temperature,  which  for  various  reasons  does  not  ex- 
actly follow  the  equator,  there  is,  as  we  have  noticed,  a 
somewhat  continuous  uprushing  current  where  the  air 
passes  upward  through  an  ascending  chimney,  which  in  a 
way  girdles  the  sea-covered  part  of  the  earth.  In  this  region 
the  movements  of  the  air  are  to  a  great  extent  under  the 
control  of  the  great  continuous  updraught.  As  we  go  to 
the  north  and  south  we  enter  realms  where  the  air  at  the 
surface  of  the  earth  is,  by  the  heat  which  it  acquires  from 
contact  with  that  surface,  more  or  less  impelled  upward; 
but  there  being  no  permanent  updraught  for  its  escape,  it 
from  time  to  time  breaks  through  the  roof  of  cold  air 
which  overlies  it  and  makes  a  temporary  channel  of  passage. 


THE  ATMOSPHERE.  107 

Going  polarward  from  the  equator,  we  first  encounter 
tliese  local  and  temporary  upcastings  of  the  air  near  the 
margin  of  the  tropical  belt.  In  these  districts,  at  least 
over  the  warmer  seas,  during  the  time  of  the  year  when  it 
is  midsummer,  and  in  the  regions  where  the  trade  winds 
are  not  strong  enough  to  sweep  the  warm  and  moisture- 
laden  air  down  to  the  equatorial  belt,  the  upward  tending 
strain  of  the  atmosphere  next  the  earth  often  becomes  so 
strong  that  the  overlying  air  is  displaced,  forming  a  chan- 
nel through  which  the  air  swiftly  passes.  As  the  moisture 
condenses  in  the  way  before  noted,  the  energy  set  free 
serves  to  accelerate  the  updraught,  and  a  hurricane  is 
begun.  At  first  the  movement  is  small  and  of  no  great 
speed,  but  as  the  amount  of  air  tending  upward  is  likely 
to  be  great,  as  is  also  the  amount  of  moisture  which  it 
contains,  the  aerial  chimney  is  rapidly  enlarged,  and  the 
speed  of  the  rising  air  increased.  The  atmosphere  next 
the  surface  of  the  sea  flows  in  toward  the  channel  of  escape; 
its  passage  is  marked  by  winds  which  are  blowing  toward 
the  centre.  On  the  periphery  of  the  movement  the  parti- 
cles move  slowly,  but  as  they  win  their  way  toward  the 
centre  they  travel  with  accelerating  velocity.  ,  On  the  prin- 
ciple which  determines  the  whirling  movement  of  the 
water  escai)ing  through  a  hole  in  the  bottom  of  a  basin, 
the  particles  of  the  air  do  not  move  on  straight  lines  toward 
the  centre,  but  journey  in  spiral  paths,  at  first  along  the 
surface,  and  then  ascending. 

We  have  noted  the  fact  that  in  a  basin  of  water  the 
direction  of  the  whirling  is  what  we  may  term  accidental — 
that  is,  dependent  on  conditions  so  slight  that  they  elude 
our  observation — ^but  in  hurricanes  a  certain  fact  deter- 
mines in  an  arbitrary  way  the  direction  in  which  the 
spin  shall  take  place.  As  soon  as  such  a  movement  of  the 
air  attains  any  considerable  diameter,  although  in  its  be- 
ginning it  may  have  spun  in  a  direction  brought  about  by 
local  accidents,  it  will  be  affected  by  the  diverse  rates  of 
travel,  by  virtue  of  the  earth's  rotation,  of  the  air  on  its 


108  OUTLINES  OF  THE  EARTH'S  HISTORY. 

equatorial  and  polar  sides.  On  the  equatorial  side  this  air 
is  moving  more  rapidly  than  it  is  on  the  polar  side.  By 
observing  the  water  passing  from  a  basin  this  principle, 
with  a  few  experiments,  can  be  made  plain.  The  result  is 
to  cause  these  great  whirlwinds  of  the  hurricanes  of  higher 
latitudes  to  whirl  round  from  right  to  left  in  the  northern 
hemisphere  and  in  the  reverse  way  in  the  southern.  The 
general  system  of  the  air  currents  still  further  affects  these, 
as  other  whirling  storms,  by  driving  their  centres  or  chim- 
neys over  the  surface  of  the  earth.  The  principle  on  which 
this  is  done  may  be  readily  understood  by  observing  how 
the  air  shaft  above  a  chimney,  through  which  we  may  ob- 
serve the  smoke  to  rise  during  a  time  of  calm,  is  drawn  off 
to  one  side  by  the  slight  current  which  exists  even  when  we 
feel  no  wind;  it  may  also  be  discerned  in  the  little  dust 
whirls  which  form  in  the  streets  on  a  summer  day  when 
the  air  is  not  much  disturbed.  While  they  spin  they  move 
on  in  the  direction  of  the  air  drift.  In  this  way  a  hurri- 
cane originating  in  the  Gulf  of  Mexico  may  gradually 
journey  under  the  influence  of  the  counter  trades  across 
the  Antilles,  or  over  southern  Florida,  and  thence  pursue 
a  devious  northerly  course,  generally  near  the  Atlantic 
coast  and  in  the  path  of  the  Gulf  Stream,  until  it  has  trav- 
elled a  thousand  miles  or  more  toward  the  North  Atlantic. 
The  farther  it  goes  northward  the  less  effectively  it  is  fed 
with  warm  and  moisture-laden  air,  the  feebler  its  move- 
ment becomes,  until  at  length  it  is  broken  up  by  the  vari- 
able winds  which  it  encounters. 

A  very  interesting  and,  from  the  point  of  view  of  the 
navigator,  important  peculiarity  of  these  whirls  is  that  at 
their  centre  there  is  a  calm,  similar  in  origin  and  nature 
to  the  calm  under  the  equator  between  the  trade-wind 
belts.  Both  these  areas  are  in  the  field  where  the  air  is 
ascending,  and  therefore  at  the  surface  of  the  earth  does 
not  affect  the  sails  of  ships,  though  if  men  ever  come  to 
use  flying  machines  and  sail  through  the  tropics  at  a  good 
height  above  the  sea  it  will  be  sensible  enough.    The  dif- 


THE  ATMOSPHERE.  109 

ference  between  the  doldrum  of  the  equator  and  that  of 
the  hurricane,  besides  their  relative  areas,  is  that  one  is 
a  belt  and  the  other  a  disk.  If  the  seafarer  happens  to  sail 
on  a  path  which  leads  him  through  the  hurricane  centre, 
he  will  first  discern,  as  from  the  untroubled  air  and  sea  he 
approaches  the  periphery  of  the  storm,  the  horizon  toward 
the  disturbance  beset  by  troubled  clouds,  all  moving  in 
one  direction.  Entering  beneath  this  pall,  he  finds  a  stead- 
ily increasing  wind,  which  in  twenty  miles  of  sailing  may, 
and  in  a  hundred  miles  surely  will,  compel  him  to  take  in 
all  but  his  storm  sails,  and  is  likely  to  bring  his  ship  into 
grave  peril.  The  most  furious  winds  the  mariner  knows 
are  those  which  he  encounters  as  he  approaches  the  still 
centre.  These  trials  are  made  the  more  appalling  by  the 
fact  that  in  the  furious  part  of  the  whirl  the  rain,  condens- 
ing from  the  ascending  air,  falls  in  torrents,  and  the  elec- 
tricity generated  in  the  condensation  gives  rise  to  vivid 
lightning.  If  the  storm-beset  ship  can  maintain  her  way, 
in  a  score  or  two  of  miles  of  journey  toward  the  centre, 
generally  very  quickly,  it  passes  into  the  calm  disk,  where 
the  winds,  blowing  upward,  cease  to  be  felt.  In  this  area 
the  ship  is  not  out  of  danger,  for  the  waves,  rolling  in  from 
the  disturbed  areas  on  either  side,  make  a  torment  of  cross 
seas,  where  it  is  hard  to  control  the  movements  of  a  sailing 
vessel  because  the  impulse  of  the  winds  is  lost.  Passing 
through  this  disk  of  calm,  the  ship  re-encounters  in  re- 
verse order  the  furious  portion  of  the  whirl,  afterward  the 
lessening  winds,  until  it  escapes  again  into  the  airs  which 
are  not  involved  in  the  great  torment. 

In  the  old  days,  before  Dove's  studies  of  storms  had 
shown  the  laws  of  hurricane  movement,  uphappy  ship- 
masters were  likely  to  be  caught  and  retained  in  hurri- 
canes, and  to  battle  with  them  for  weeks  until  their  vessels 
were  beaten  to  pieces.  Now  the  "  Sailing  Directions," 
which  are  the  mariner's  guide,  enable  him,  from  the  direc- 
tion of  the  winds  and  the  known  laws  of  motion  of  the 
storm  centre,  to  sail  out  of  the  danger,  so  that  in  most 


110  OUTLINES  OF  THE  EARTH'S  HISTORY. 

cases  he  may  escape  calamity.  It  is  otherwise  with  the 
people  who  dwell  upon  the  land  over  which  these  atmos- 
pheric convulsions  sweep.  Fortunately,  where  these  great 
whirlwinds  trespass  on  the  continent,  they  quickly  die  out, 
because  of  the  relative  lack  of  moisture  which  serves  to 
stimulate  the  uprush  which  creates  them.  Thus  in  their 
more  violent  forms  hurricanes  are  only  felt  near  the  sea, 
and  generally  on  islands  and  peninsulas.  There  the  hurri- 
cane winds,  by  the  swiftness  of  their  movement,  which 
often  attains  a  speed  of  a  hundred  miles  or  more,  apply 
a  great  deal  of  energy  to  all  obstacles  in  their  path.  The 
pressure  thus  produced  is  only  less  destructive  than  that 
which  is  brought  about  by  the  tornadoes,  which  are  next 
to  be  described. 

There  is  another  effect  from  hurricanes  which. is  even 
more  destructive  to  life  than  that  caused  by  the  direct 
action  of  the  wind.  In  these  whirlings  great  differences  in 
atmospheric  pressure  are  brought  about  in  contiguous 
areas  of  sea.  The  result  is  a  sudden  elevation  in  the  level 
of  one  part  of  the  water.  These  disturbances,  where  the 
shore  lands  are  low  and  thickly  peopled,  as  is  the  case  along 
the  western  coast  of  the  Bay  of  Bengal,  may  produce  in- 
undations which  are  terribly  destructive  to  life  and  prop- 
erty. They  are  known  also  in  southern  Florida  and  along 
the  islands  of  the  Caribbean,  but  in  that  region  are  not 
so  often  damaging  to  mankind. 

Fortunately,  hurricanes  are  limited  to  a  very  small  part 
of  the  tropical  district.  They  occur  only  in  those  regions, 
on  the  eastern  faces  of  tropical  lands,  where  the  general 
westerly  set  of  the  winds  favours  the  accumulation  of  great 
bodies  of  very  warm,  moist  air  next  the  surface  of  the 
•sea.  The  western  portion  of  the  Gulf  of  Mexico  and  the 
Caribbean,  the  Bay  of  Bengal,  and  the  southeastern  por- 
tion of  Asia  are  especially  liable  to  their  visitations.  They 
sometimes  develop,  though  with  less  fury,  in  other  parts 
of  the  tropics.  On  the  western  coast  of  South  America 
and  Africa,  where  the  oceans  are  visited  by  the  dry  land 


THE  ATMOSPHERE.  HI 

winds,  and  where  the  waters  are  cooled  by  currents  setting 
in  from  high  latitudes,  they  are  unknown. 

Only  less  in  order  of  magnitude  than  the  hurricanes 
are  the  circular  storms  known  as  cyclones.  These  occur 
on  the  continents,  especially  where  they  afford  broad  plains 
little  interrupted  by  mountain  ranges.  They  are  particu- 
larly well  exhibited  in  that  part  of  North  America  north 
of  Mexico  and  south  of  Hudson  Bay.  Like  the  hurricanes, 
they  appear  to  be  due  to  the  inrush  of  relatively  w^arm 
air  entering  an  updraught  which  had  been  formed  in  the 
overlying,  cooler  portions  of  the  atmosphere.  They  are, 
however,  much  less  energetic,  and  often  of  greater  size 
than  the  hurricane  whirl.  The  lack  of  energy  is  probably 
due  to  the  comparative  dryness  of  the  air.  The  greater 
width  of  the  ascending  column  may  perhaps  be  accounted 
for  by  the  fact  that,  originating  at  a  considerable  height 
above  the  sea,  they  have  a  less  thickness  of  air  to  break 
through,  and  so  the  upward  setting  column  is  readily 
made  broad. 

The  cyclones  of  North  America  appear  generally  to 
originate  in  the  region  of  the  Rocky  Mountains,  though  it 
is  probable  that  in  some  instances,  perhaps  in  many,  the' 
upward  set  of  the  air  which  begins  the  storm  originates 
in  the  ocean  along  the  Pacific  coast.  They  gather  energy 
as  they  descend  the  great  sloping  plain  leading  eastward 
from  the  Eocky  ^fountains  to  the  central  portion  of  the 
great  continental  valley.  Thence  they  move  on  across 
the  country  to  the  Atlantic  coast.  Not  infrequently  they 
continue  on  over  the  ocean  to  the  European  continent. 
The  eastward  passage  of  the  storm  centre  is  due  to  the  pre- 
vailing eastward  movement  of  the  air  in  its  upper  part 
throughout  that  portion  of  the  northern  hemisphere. 
Commonly  they  incline  somewhat  to  the  northward  of  east 
in  their  journey.  In  all  cases  the  winds  appear  to  blow 
spirally  into  the  common  storm  centre.  There  is  the  same 
doldrum  area  or  calm  field  in  the  centre  of  the  storm  that 
we  note  between  the  trade  winds  and  in  the  middle  of  a 


112  OUTLINES  OF  THE  EARTH'S  HISTORY. 

hurricane  disk,  though  this  area  is  less  defined  than  in 
the  other  instances,  and  the  forward  motion  of  tlie  storm 
at  a  considerable  speed  is  in  most  cases  characteristic  of 
the  disturbance.  On  the  front  of  one  of  these  storms  in 
North  America  the  winds  commonly  begin  in  the  north- 
east, thence  they  veer  by  the  east  to  the  southwest.  At 
this  stage  in  the  movement  the  storm  centre  has  passed 
by,  the  rainfall  commonly  ceases,  and  cold,  dry  winds  set- 
ting to  the  northwestward  set  in.  This  is  caused  by  the 
fact  that  the  ascending  air,  having  attained  a  height  above 
the  earth,  settles  down  behind  the  storm,  forming  an  anti- 
cyclone or  mass  of  dry  air,  which  presses  against  the  re- 
treating side  of  the  great  whirlwind. 

In  front  of  the  storm  the  warm  and  generally  moist 
relatively  warm  air,  pressing  in  toward  the  point  of  uprise 
and  overlaid  by  the  upper  cold  air,  is  brought  into  a  con- 
dition where  it  tends  to  form  small  subordinate  shafts  up 
through  which  it  whirls  on  the  same  principle,  but  with 
far  greater  intensity  than  the  main  ascending  column. 
The  reason  for  the  violence  of  this  movement  is  that  the 
difference  in  temperature  of  the  air  next  the  surface  and 
that  at  the  height  of  a  few  thousand  feet  is  great.  As 
might  be  expected,  these  local  spinnings  are  most  apt  to 
occur  in  the  season  when  the  air  next  the  earth  is  rela- 
tively warm,  and  they  are  aptest  to  take  place  in  the  half 
of  the  advancing  front  lying  between  the  east  and  south, 
for  the  reason  that  there  the  highest  temperatures  and  the 
greatest  humidity  are  likely  to  coexist.  In  that  part  of  the 
field,  during  the  time  when  the  storm  is  advancing  from 
the  Eocky  Mountains  to  the  Atlantic,  a  dozen  or  more  of 
these  spinning  uprushes  may  be  produced,  though  few  of 
them  are  likely  to  be  of  large  size  or  of  great  intensity. 

The  secondary  storms  of  cyclones,  such  as  are  above 
noted,  receive  the  name  of  tornadoes.  They  are  frequent 
and  terrible  visitations  of  the  country  from  northern 
Texas,  Florida,  and  Alabama  to  about  the  line  of  the 
Great  Lakes;  they  are  rarely  developed  in  the  region  west 


THE  ATMOSPHERE.  113 

of  central  Kansas,  and  only  occasionally  do  they  exhibit 
much  energy  in  the  region  east  of  the  plain-lands  of  the 
Ohio  Valley.  Although  known  in  other  lands,  they  no- 
where, so  far  as  our  observations  go,  exhibit  the  paroxys- 
mal intensity  which  they  show  in  the  central  portion  of 
the  North  American  continent.  There  the  air  which  they 
affect  acquires  a  speed  of  movement  and  a  fury  of  action 
unknown  in  any  other  atmospheric  disturbances,  even  in 
those  of  the  hurricanes. 

The  observer  who  has  a  chance  to  note  from  an  advan- 
tageous position  the  development  of  a  tornado  observes 
that  in  a  tolerably  still  air,  or  at  least  an  air  unaifected  by 
violent  winds — generally  in  what  is  termed  a  "  sultry  " 
state  of  the  atmosphere — the  storm  clouds  in  the  distance 
begin  to  form  a  kind  of  funnel-shaped  dependence,  which 
gradually  extends  until  it  appears  to  touch  the  earth.  As 
the  clouds  are  low,  this  downward-growing  column  prob- 
ably in  no  case  is  observed  for  the  height  of  more  than 
three  or  four  thousand  feet.  As  the  funnel  descends,  the 
clouds  above  and  about  it  may  be  seen  to  take  on  a  whirl- 
ing movement  around  the  centre,  and  under  favourable 
circumstances  an  uprush  of  vapours  may  be  noted  in  the 
centre  of  the  swaying  shaft.  As  the  whirl  comes  nearer, 
the  roar  of  the  disturbance,  which  at  a  distance  is  often 
compared  to  the  sound  made  by  a  threshing  machine  or 
to  that  of  distant  musketry,  increases  in  loudness  until  it 
becomes  overwhelming.  When  a  storm  such  as  this  strikes 
a  building,  it  is  not  only  likely  to  be  razed  by  the  force 
of  the  wind,  but  it  may  be  exploded,  as  by  the  action  of 
gunpowder  fired  within  its  walls,  through  the  sudden  ex- 
pansion of  the  air  which  it  contains.  In  the  centre  of  the 
column,  although  it  rarely  has  a  diameter  of  more  than 
a  few  hundred  feet,  the  uprush  is  so  swift  that  it  makes 
a  partial  vacuum.  The  air,  striving  to  get  into  the  space 
which  it  is  eager  to  occupy,  is  whirling  about  at  such  a 
rate  that  the  centrifugal  motion  which  it  thus  acquires 
restrains  its  entrance.     In  this  way  there  may  be,  as  the 


114 


OUTLINES  OF  THE  EARTH'S  HISTORY. 


column  rapidly  moves  by,  a  difference  of  pressure  amount- 
ing probably  to  what  the  mercury  of  a  barometer  would 
indicate  by  four  or  five  inches  of  fall.  Unless  the  structure 
is  small  and  its  walls  strong,  its  roof  and  sides  are  apt  to 
be  blown  apart  by  this  difference  of  pressure  and  the  con- 
sequent expansion  of  the  contained  air.  In  some  eases 
where  wooden  buildings  have  withstood  this  curious  action 
the  outer  clapboards  have  been  blown  off  by  the  expansion 
of  the  small  amount  of  air  contained  in  the  interspaces 
between  that  covering  and  the  lath  and  plaster  within 
(see  Fig.  9). 


m 

t^'—^^- 

-■'*il             '-■   \:^_-J^~L__ 

a..  « 

1. 

-WMM 

^^B^^m 

=^i& 

-a 

"^^^^E^m 

^m^:  i 

'^'^■^mmg' 

=;-- 

H^ 

^jraj          ,ra; 

.-i=j 

==jilp=«=Eii^lBEL 

"    !;»; 

'  r                ~  ~~-^ 

a 

^^^^fe 

_ .  -^ 

ji      ---^i^^ 

Fig.  9. — Showing  effect  of  expansion  of  air  contained  in  a  hollow 
wall  during  the  passage  of  the  storm. 


The  blow  of  the  air  due  to  its  rotative  whirling  has  in 
several  cases  proved  sufficient  to  throw  a  heavy  locomotive 
from  the  track  of  a  well-constructed  railway.  In  all  cases 
where  it  is  intense  it  will  overturn  the  strongest  trees. 
The  ascending  wind  in  the  centre  of  the  column  may 
sometimes  lift  the  bodies  of  men  and  of  animals,  as  well 
as  the  branches  and  trunks  of  trees  and  the  timber  of 
houses,  to  the  height  of  hundreds  of  feet  above  the  sur- 
face.    One  of  the  most  striking  exhibitions  of  the  upsucking 


THE  ATMOSPHERE.  115 

action  in  a  tornado  is  afforded  by  the  effect  which  it  pro- 
duces when  it  crosses  a  small  sheet  of  water.  In  certain 
cases  where,  in  the  Northwestern  States  of  this  country,  the 
path  of  the  storm  lay  over  the  pool,  the  whole  of  the  water 
from  a  basin  acres  in  extent  has  been  entirely  carried  away, 
leaving  the  surface,  as  described  by  an  observer,  apparently 
dry  enough  to  plough. 

Fortunately  for  the  interests  of  man,  as  well  as  those 
of  the  lower  organic  life,  the  paths  of  these  storms,  or  at 
least  the  portion  of  their  track  where  the  violence  of  the 
air  movement  makes  them  very  destructive,  often  does  not 
exceed  five  hundred  feet  in  width,  and  is  rarely  as  great  as 
half  a  mile  in  diameter.  In  most  cases  the  length  of  the 
journey  of  an  individual  tornado  does  not  exceed  thirty 
miles.    It  rarely  if  ever  amounts  to  twice  that  distance. 

In  every  regard  except  their  small  size  and  their  vio- 
lence these  tornadoes  closely  resemble  hurricanes.  There 
is  the  same  broad  disk  of  air  next  the  surface  spirally  re- 
volving toward  the  ascending  centre,  where  its  motion 
is  rapidly  changed  from  a  horizontal  to  a  vertical  direc- 
tion. The  energy  of  the  uprush  in  both  cases  is  increased 
by  the  energy  set  free  through  the  condensation  of  the 
water,  which  tends  further  to  heat  and  thus  to  expand  the 
air.  The  smaller  size  of  the  tornado  may  be  accounted  for 
by  the  fact  that  we  have  in  their  originating  conditions 
a  relatively  thin  layer  of  warm,  moist  air  next  the  earth 
and  a  relatively  very  cold  layer  immediately  overlying  it. 
Thus  the  tension  which  serves  to  start  the  movement  is 
intense,  though  the  masses  involved  are  not  very  great. 
The  short  life  of  a  tornado  may  be  explained  by  the  fact 
that,  though  it  apparently  tends  to  grow  in  width  and 
energy,  the  central  spout  is  small,  and  is  apt  to  be  broken 
by  the  movements  of  the  atmosphere,  which  in  the  front 
of  a  cyclone  are  in  all  cases  irregular. 

On  the  warmer  seas,  but  often  beyond  the  limits  of  the 
tropics,  another  class  of  spinning  storms,  known  as  water- 
spouts, may  often  be  observed.     In  general  appearance 


116  OUTLINES  OF  THE  EARTH'S  HISTORY. 

these  air  whirls  resemble  tornadoes,  except  that  they  are 
in  all  cases  smaller  than  that  group  of  whirlings.  As  in 
the  tornadoes,  the  waterspout  begins  with  a  funnel,  which 
descends  from  the  sky  to  the  surface  of  the  sea.  Up  the 
tube  vapours  may  be  seen  ascending  at  great  speed,  the 
whole  appearing  like  a  gigantic  pillar  of  swiftly  revolving 
smoke.  When  the  whirl  reaches  the  water,  it  is  said  that 
the  fluid  leaps  up  into  the  tube  in  the  form  of  dense  spray, 
an  assertion  which,  in  view  of  the  fact  of  the  action  of 
a  tornado  on  a  lake  as  before  described,  may  well  be  be- 
lieved. Like  the  tornadoes  and  dust  whirls,  the  life  of  a 
waterspout  appears  to  be  brief.  They  rarely  endure  for 
more  than  a  few  minutes,  or  journey  over  the  sea  for 
more  than  two  or  three  miles  before  the  column  appears 
to  be  broken  by  some  swaying  of  the  atmosphere.  As 
these  peculiar  storms  are  likely  to  damage  ships,  the  old- 
fashioned  sailors  were  accustomed  to  fire  at  them  with 
cannon.  It  has  been  claimed  that  a  shot  would  break  the 
tube  and  end  the  little  convulsion.  This,  in  view  of  the 
fact  that  they  appear  to  be  easily  broken  up  by  relatively 
trifling  air  currents,  may  readily  be  believed.  The  danger 
which  these  disturbances  bring  to  ships  is  probably  not 
very  serious. 

The  special  atmospheric  conditions  which  bring  about 
the  formation  of  waterspouts  are  not  well  known;  they 
doubtless  include,  however,  warm,  moist  air  next  the  sur- 
face of  the  sea  and  cold  air  above.  Just  why  these  storms 
never  attain  greater  size  or  endurance  is  not  yet  known. 
These  disturbances  have  been  seen  for  centuries,  but  as 
yet  they  have  not  been,  in  the  scientific  sense,  observed. 
Their  picturesqueness  attracts  all  beholders;  it  is  interest- 
ing to  note  the  fact  that  perhaps  the  earliest  description 
of  their  phenomena — one  which  takes  account  in  the  sci- 
entific spirit  of  all  the  features  which  they  present — was 
written  by  the  poet  Camoens  in  the  Lusiad,  in  which  he 
strangely  mingles  fancy  and  observation  in  his  account 
of  the  great  voyage  of  Yasco  da  Gama,     The  poet  even 


THE  ATMOSPHERE.  117 

notes  that  the  water  which  falls  when  the  spout  is  broken 
is  not  salt,  but  fresh — a  point  which  clearly  proves  that 
not  much  of  the  water  which  the  tube  contains  is  derived 
from  the  sea.  It  is,  in  fact,  watery  vapour  drawn  from  the 
air  next  the  surface  of  the  ocean,  and  condensed  in  its 
ascent  through  the  tube.  In  this  and  other  descriptions 
of  Nature  Camoens  shows  more  of  the  scientific  spirit  than 
any  other  poet  of  his  time.  He  was  in  this  regard  the  first 
of  modern  writers  to  combine  a  spiritual  admiration  for 
Nature  with  some  sense  of  its  scientific  meaning. 

In  treating  of  the  atmosphere,  meteorologists  base  their 
studies  largely  on  changes  in  the  weight  of  that  medium, 
which  they  determine  by  barometric  observations.  In  fact, 
the  science  of  the  air  had  its  beginning  in  Pascal's  admira- 
ble observation  on  the  changes  in  the  height  of  a  column 
of  mercury  contained  in  a  bent  tube  as  he  ascended  the 
volcanic  peak  known  as  Puy  de  Dome,  in  central  France. 
As  before  noted,  it  is  to  the  disturbances  in  the  weight 
of  the  air,  brought  about  mainly  by  variations  in  tempera- 
ture, that  we  owe  all  its  currents,  and  it  is  upon  these 
winds  that  the  features  we  term  climate  in  largest  measure 
depend.  Every  movement  of  the  winds  is  not  only  brought 
about  by  changes  in  the  relative  weight  of  the  air  at  cer- 
tain points,  but  the  winds  themselves,  owing  to  the  mo- 
mentum which  the  air  attains  by  them,  serve  to  bring 
about  alterations  in  the  quantity  of  air  over  different  parts 
of  the  earth,  which  are  marked  most  distinctly  by  baro- 
metric variations.  These  changes  are  exceedingly  com- 
plicated; a  full  account  of  them  would  demand  the  space 
of  this  volume.  A  few  of  the  facts,  however,  should  be 
presented  here.  In  the  first  place,  we  note  that  each  day 
there  is  normally  a  range  in  the  pressure  which  causes  the 
barometer  to  be  at  the  lowest  at  about  four  o'clock  in  the 
morning  and  four  o'clock  in  the  afternoon,  and  highest  at 
about  ten  o'clock  in  those  divisions  of  the  day.  This 
change  is  supposed  to  be  due  to  the  fact  that  the  motes 
of  dust  in  the  atmosphere  in  the  night,  becoming  cooled, 


118  OUTLINES  OF  THE  EARTH'S  HISTORY. 

condense  the  water  vapour  upon  their  surfaces,  thus  dimin- 
ishing the  volume  of  the  air.  When  the  sun  rises  the  water 
evaporated  by  the  heat  returns  from  these  little  store- 
houses into  the  body  of  the  atmosphere.  Again  in  the 
evening  the  condensation  sets  in;  at  the  same  time  the  air 
tends  to  drift  in  from  the  region  to  the  westward,  where 
the  sun  is  still  high,  toward  the  field  where  the  barometer 
has  been  thus  lowered;  the  current  gradually  attains  a 
certain  volume,  and  so  brings  about  the  rise  of  the  barome- 
ter about  ten  o^clock  at  night. 

In  the  winter  time,  particularly  on  the  well-detached 
continent  of  North  America,  we  find  a  prevailing  high 
barometer  in  the  interior  of  the  country  and  a  correspond- 
ing low  state  of  pressure  on  the  Atlantic  Ocean.  In  the 
summer  season  these  conditions  are  on  the  whole  reversed. 

Under  the  tropics,  in  the  doldrum  belt,  there  is  a  zone 
of  low  barometer  connected  to  the  ascending  currents 
which"  take  place  along  that  line.  This  is  a  continuous 
manifestation  of  the  same  action  which  gives  a  large  area 
of  a  disklike  form  in  the  centre  or  eye  of  the  hurricane 
and  in  the  middle  portion  of  the  tornado's  whirl.  In  gen- 
eral, it  may  be  said  that  the  weight  of  the  air  is  greatest 
in  the  regions  from  which  it  is  blowing  toward  the  points 
of  upward  escape,  and  least  in  and  about  those  places  where 
the  superincumbent  air  is  rising  through  a  temporary  or 
permanent  line  of  escape.  In  other  words,  ascending  air 
means  generally  a  relatively  low  barometer,  while  descend- 
ing air  is  accompanied  by  greater  pressure  in  the  field  upon 
which  it  falls. 

In  almost  every  part  of  the  earth  which  is  affected  by 
a  particular  physiography  we  find  that  the  movements  of 
the  atmosphere  next  the  surface  are  qualified  by  the  con- 
dition which  it  encounters.  In  fact,  if  a  person  were  pos- 
sessed of  all  the  knowledge  which  could  be  obtained  con- 
cerning winds,  he  could  probably  determine  as  by  a  map 
the  place  where  he  might  chance  to  find  himself,  provided 
he  could  extend  his  observations  over  a  term  of  years. 


THE  ATMOSPHERE.  119 

In  other  words,  the  regimen  of  the  winds — at  least  those 
of  a  superficial  nature — is  almost  as  characteristic  of  the 
field  over  which  they  go  as  is  a  map  of  the  country.  Of 
these  special  winds  a  number  of  the  more  important  have 
been  noted,  only  a  few  of  which  we  can  advert  to.  First 
among  these  may  well  come  the  land  and  sea  breezes  which 
are  remarked  about  all  islands  which  are  not  continuously 
swept  by  permanent  winds.  One  of  the  most  characteristic 
instances  of  these  alternate  winds  is  perhaps  that  afforded 
on  the  island  of  Jamaica. 

The  island  of  Jamaica  is  so  situated  within  the  basin 
of  the  Caribbean  that  it  does  not  feel  the  full  influence 
of  the  trades.  It  has  a  range  of  high  mountains  through 
its  middle  part.  In  the  daytime  the  surface  of  the  land, 
which  has  the  sun  overhead  twice  each  year,  and  is  always 
exposed  to  nearly  vertical  radiation,  becomes  intensely 
hot,  so  that  an  upcurrent  is  formed.  The  formation  of  this 
current  is  favoured  by  the  mountains,  which  apply  a  part 
of  the  heat  at  the  height  of  about  a  mile  above  the  surface 
of  the  sea.  This  action  is  parallel  to  that  we  notice  when, 
in  order  to  create  a  draught  in  the  air  of  a  chimney,  we 
put  a  torch  some  distance  up  above  the  fireplace,  thus 
diminishing  the  height  of  the  column  of  air  which  has 
to  be  set  in  motion.  It  is  further  shown  by  the  fact  that 
when  miners  sought  to  make  an  upcurrent  in  a  shaft,  in 
order  to  lead  pure  air  into  the  workings  through  other 
openings,  they  found  after  much  experience  that  it  was 
better  to  have  the  fire  near  the  top  of  the  shaft  rather 
than  at  the  bottom. 

The  ascending  current  being  induced  up  the  mountain 
sides  of  Jamaica,  the  air  is  forced  in  from  the  sea  to  the 
relatively  free  space.  Before  noon  the  current,  aided  in 
its  speed  by  a  certain  amount  of  the  condensation  of  the 
watery  vapour  before  described,  attains  the  proportions  of 
a  strong  wind.  As  the  sun  begins  to  sink,  the  earth's  sur- 
face pours  forth  its  heat;  the  radiation  being  assisted  by 
the  extended  surfaces  of  the  plants,  cooling  rapidly  takes 
9 


120  OUTLINES  OF  THE  EARTH'S  HISTORY. 

place.  Meanwhile  the  sea,  because  of  the  great  heat-stor- 
ing power  of  water,  is  very  little  cooled,  the  ascent  of  the 
air  ceases,  the  temporary  chimney  with  its  updraught  is 
replaced  by  a  downward  current,  and  the  winds  blow  from 
the  land  until  the  sun  comes  again  to  reverse  the  current. 
In  many  cases  these  movements  of  the  daily  winds  flowing 
into  and  from  islands  induce  a  certain  precipitation  of 
moisture  in  the  form  of  rain.  Generally,  however,  their 
effect  is  merely  to  ameliorate  the  heat  by  bringing  alter- 
nately currents  from  the  relatively  cool  sea  and  from  the 
upper  atmosphere  to  lessen  the  otherwise  excessive  tem- 
perature of  the  fields  which  they  traverse. 

Although  characteristic  sea  and  land  winds  are  limited 
to  regions  where  the  sun's  heat  is  great,  they  are  traceable 
even  in  high  latitudes  during  the  periods  of  long-continued 
calm  attended  with  clear  skies.  Thus  on  the  island  of 
Martha's  Vineyard,  in  Massachusetts,  the  writer  has  noted, 
when  the  atmosphere  was  in  such  a  state,  distinct  night 
and  day,  or  sea  and  land,  breezes  coming  in  their  regular 
alternation.  During  the  night  when  these  alternate  winds 
prevail  the  central  portion  of  the  island,  at  the  distance 
of  three  miles  from  the  sea,  is  remarkably  cold,  the  low 
temperature  being  due  to  the  descending  air  current.  To 
the  same  physical  cause  may  be  attributed  the  frequent 
insets  of  the  sea  winds  toward  midday  along  the  conti- 
nental shores  of  various  countries.  Thus  along  the  coast 
of  New  England  in  the  summer  season  a  clear,  still,  hot 
day  is  certain  to  lead  to  the  creation  of  an  ingoing  tide 
of  air,  which  reaches  some  miles  into  the  interior.  This 
stream  from  the  sea  enters  as  a  thin  wedge,  it  often  being 
possible  to  note  next  the  shore  when  the  movement  begins 
a  difference  of  ten  degrees  of  temperature  between  the  sur- 
face of  the  ground  to  which  the  point  of  the  wedge  has 
attained,  and  a  position  twenty  feet  higher  in  the  air.  This 
is  a  beautiful  example  to  show  at  once  how  the  relative 
weight  of  the  atmosphere,  even  when  the  differences  are 
slight,  may  bring  about  motion,  and  also  how  masses  of  the 


THE  ATMOSPHEKE.  121 

atmosphere  may  move  by  or  through  the  rest  of  the  medi- 
um in  a  way  which  w^e  do  not  readily  conceive  from  our 
observations  on  the  transparent  mass.  Very  few  people 
have  any  idea  how  general  is  the  truth  that  the  air,  even 
in  continuous  winds,  tends  to  move  in  more  or  less  indi- 
vidualized masses.  This,  however,  is  made  very  evident 
hy  watching  the  gusts  of  a  storm  or  the  wandering  patches 
of  wind  which  disturb  the  surface  of  an  otherwise  smooth 
sea. 

Among  the  notable  local  winds  are  those  which  from 
their  likeness  to  the  Fohn  of  the  Swiss  valleys  receive 
that  name.  Folms  are  produced  where  a  body  of  air  blowing 
against  the  slope  of  a  continuous  mountain  range  is  lifted 
to  a  considerable  height,  and,  on  passing  over  the  crest, 
falls  again  to  a  low  position.  In  its  ascent  the  air  is  cooled, 
rarefied,  and  to  a  great  extent  deprived  of  its  moisture. 
In  descending  it  is  recondensed,  and  by  the  process  by 
which  its  atoms  are  brought  together  its  latent  heat  is 
made  sensible.  There  being  but  little  watery  vapour  in 
the  mass,  this  heat  is  not  much  called  for  by  that  heat- 
storing  fluid,  and  so  the  air  is  warmed.  So  far  Fohn  winds 
have  only  been  remarked  as  conspicuous  features  in  Swit- 
zerland and  on  the  eastern  face  of  the  Rocky  Mountains. 
In  the  region  about  the  head  waters  of  the  Missouri  and 
to  the  northward  their  influence  in  what  are  called  the 
Chinook  winds  is  distinctly  to  ameliorate  the  severe  win- 
ter climate  of  the  country. 

In  almost  all  great  desert  regions,  particularly  in  the 
typical  Sahara,  we  find  a  variety  of  storm  belonging  to  the 
whirlwind  group,  which,  owing  to  the  nature  of  the  coun- 
try, take  on  special  characteristics.  These  desert  storms 
take  up  from  the  verdureless  earth  great  quantities  of  sand 
and  other  fine  debris,  which  often  so  clouds  the  air  as  to 
bring  the  darkness  of  night  at  midday.  Their  whirlings 
appear  in  size  to  be  greater  than  those  which  produce 
tornadoes  or  waterspouts,  but  less  than  hurricanes  or  cy- 
clones.    Little,  however,  is  known  about  them.     They 


122  OUTLINES  OF  THE  EARTH'S  HISTORY. 

have  not  been  well  observed  by  meteorologists.  In  some 
ways  they  are  important,  for  the  reason  that  they  serve 
to  carry  the  desert  sand  into  regions  previously  verdure- 
clad,  and  thus  to  extend  the  bounds  of  the  desolate  fields 
in  which  they  originate.  Where  they  blow  off  to  the  sea- 
ward, they  convey  large  quantities  of  dust  into  the  ocean, 
and  thus  serve  to  wear  down  the  surface  of  the  land  in. 
regions  where  there  are  no  rivers  to  effect  that  action  in 
the  normal  way. 

Notwithstanding  its  swift  motion  when  impelled  by 
differences  in  weight,  the  movements  of  the  air  have  had 
but  little  direct  and  immediate  influence  on  the  surface  of 
the  earth.  The  greater  part  of  the  work  which  it  does,  as 
we  shall  see  hereafter,  is  done  through  the  waters  which 
it  impels  and  bears  about.  Yet  where  winds  blow  over 
verdureless  surfaces  the  effect  of  the  sand  which  they 
sweep  before  them  is  often  considerable.  In  regions  of 
arid  mountains  the  winds  often  drive  trains  of  sand 
through  the  valleys,  where  the  sharp  particles  cut  the 
rocks  almost  as  effectively  as  torrents  of  water  would,  dis- 
tributing the  wearing  over  the  width  of  the  valley.  The 
dust  thus  blown  from  a  desert  region  may,  when  it  attains 
a  country  covered  with  vegetation,  gradually  accumulate 
on  its  surface,  forming  very  thick  deposits.  Thus  in  north- 
western China  there  is  a  wide  area  where  dust  accumula- 
tions blown  from  the  arid  districts  of  central  Asia  have 
gradually  heaped  up  in  the  course  of  ages  to  the  depth  of 
thousands  of  feet,  and  this  although  much  of  the  dehris 
is  continually  being  borne  away  by  the  action  of  the  rain 
waters  as  they  journey  toward  the  sea.  Such  dust  accumu- 
lations occur  in  other  parts  of  tlie  world,  particularly  in 
the  districts  about  the  upper  Mississippi  and  in  the  valleys 
of  the  Rocky  Mountains,  but  nowhere  are  they  so  conspicu- 
ous as  in  the  region  first  mentioned. 

Where  prevailing  winds  from  the  sea,  from  great  lakes, 
and  even  from  considerable  rivers,  blow  against  sandy 
shores  or  cliffs  of  the  same  nature,  large  quantities  of  sand 


THE  ATMOSPHEKE.  123 

and  dust  are  often  driven  inland  from  the  coast  line.  In 
most  cases  these  wind-borne  materials  take  on  the  form 
of  dunes,  or  heaps  of  sand/ varying  from  a  few  feet  to  sev- 
eral hundred  feet  in  height.  It  is  characteristic  of  these 
hills  of  blown  sand  that  they  move  across  the  face  of  the 
country.  Under  favourable  conditions  they  may  journey 
scores  of  miles  from  the  shore.  The  marching  of  a  dune 
is  effected  through  the  rolling  up  of  the  sand  on  the  wind- 
ward side  of  the  elevation,  when  it  is  impelled  by  the  cur- 
rent of  air  to  the  crest  where  it  falls  into  the  lee  or  shelter 
which  the  hill  makes  to  the  wind.  In  this  way  in  the 
course  of  a  day  the  centre  of  the  dune,  if  the  wind  be 
blowing  furiously,  may  advance  a  measurable  distance 
from  the  place  it  occupied  before.  By  fits  and  starts  this 
ongoing  may  be  indefinitely  continued.  A  notable  and 
picturesque  instance  of  the  march  of  a  great  dune  may 
be  had  from  the  case  in  which  one  of  them  overwhelmed 
in  the  last  century  the  village  of  Eccles  in  southeastern 
England.  The  advancing  sand  gradually  crept  into  the 
hamlet,  and  in  the  course  of  a  decade  dispossessed  the 
people  by  burying  their  houses.  In  time  the  summit  of 
the  church  spire  disappeared  from  view,  and  for  many 
years  thereafter  all  trace  of  the  hamlet  was  lost.  Of  late 
years,  however,  the  onward  march  of  the  sands  has  dis- 
closed the  church  spire,  and  in  the  course  of  another  cen- 
tury the  place  may  be  revealed  on  its  original  site,  un- 
changed except  that  the  marching  hill  will  be  on  its  other 
side. 

In  the  region  about  the  head  of  the  Bay  of  Biscay 
the  quantity  of  these  marching  sands  is  so  great  that  at 
one  time  they  jeopardized  the  agriculture  of  a  large  dis- 
trict. The  French  Government  has  now  succeeded,  by 
carefully  planting  the  surface  of  the  country  with  grasses 
and  other  herbs  which  will  grow  in  such  places,  in  check- 
ing the  movement  of  the  wind-blown  materials.  By  so 
doing  they  have  merely  hastened  the  process  by  which 
Nature  arrests  the  march  of  dunes.    As  these  heaps  creep 


124  OUTLINES  OF  THE  EARTH'S  HISTORY. 

away  from  the  sea,  they  generally  come  into  regions  where 
a  greater  variety  of  plants  flourish;  moreover,  their  sand 
grains  become  decayed,  so  that  they  afford  a  better  soil. 
Gradually  the  mat  of  vegetation  binds  them  down,  and  in 
time  covers  them  over  so  that  only  the  expert  eye  can 
recognise  their  true  nature.  Only  in  desert  regions  can  the 
march  of  these  heaps  be  maintained  for  great  distances. 

Characteristic  dunes  occur  from  point  to  point  all 
along  the  Atlantic  coast  from  the  State  of  Maine  to  the 
northern  coast  of  Florida.  They  also  occur  along  the 
coasts  of  our  Great  Lakes,  being  particularly  well  devel- 
oped at  the  southern  end  of  Lake  Michigan,  where  they 
form,  perhaps,  the  most  notable  accumulations  within  the 
limits  of  the  L^nited  States. 

When  blown  sands  invade  a  forest  and  the  deposit  is 
rapidly  accumulated,  the  trees  are  often  buried  in  an  un- 
decayed  condition.  In  this  state,  with  certain  chemical 
reactions  which  may  take  place  in  the  mass,  the  woody 
matter  is  apt  to  become  replaced  by  silex  dissolved  from 
the  sand,  which  penetrates  the  tissues  of  the  plants.  In 
this  way  salicified  forests  are  produced,  such  as  are  found 
in  the  region  of  the  Kocky  Mountains,  where  the  trunks 
of  the  trees,  now  very  hard  stone,  so  perfectly  preserve 
their  original  structure  that  when  cut  and  polished  they 
may  be  used  for  decorative  purposes.  Conspicuous  as  is 
this  work  of  the  dunes,  it  is  in  a  geological  way  much  less 
important  than  that  accomplished  by  the  finer  dust  which 
drifts  from  one  region  of  land  to  another  or  into  the  sea. 
Because  of  their  weight,  the  sand  grains  journey  over  the 
surface  of  the  earth,  except,  indeed,  where  they  are  up- 
lifted by  whirl  ?torms.  They  thus  can  not  travel  very  fast 
or  far.  Dust,  however,  rises  into  the  air,  and  journeys  for 
indefinite  distances.  We  thus  see  how  slight  differences  in 
the  weight  of  substances  may  profoundly  affect  the  condi- 
tions of  their  deportation. 


THE  ATMOSPHERE.  125 

The  System  of  Waters. 

The  envelope  of  air  wraps  the  earth  completely  about, 
and,  though  varying  in  thickness,  is  everywhere  present 
over  its  surface.  That  of  the  waters  is  much  less  equally 
distributed.  Because  of  its  weight,  it  is  mainly  gathered 
in  the  depths  of  the  earth,  where  it  lies  in  the  interstices 
of  the  rocks  and  in  the  great  realm  of  the  seas.  Only  a 
very  small  portion  of  the  fluid  is  in  the  atmosphere  or  on 
the  land.  Perhaps  less  than  a  ten  thousandth  part  of  the 
whole  is  at  any  one  time  on  this  round  from  the  seas 
through  the  air  to  the  land  and  back  to  the  great  reservoir. 

The  great  water  store  of  the  earth  is  contained  in  two 
distinct  realms — in  the  oceans,  where  the  fluid  is  concen- 
trated in  a  quantity  which  fills  something  like  nine  tenths 
of  the  hollows  formed  by  the  corrugations  of  the  earth's 
surface;  and  in  the  rocks,  where  it  is  stored  in  a  finely 
divided  form,  partly  between  the  grains  of  the  stony  matter 
and  partly  in  the  substance  of  its  crystals,  where  it  exists 
in  a  combination,  the  precise  nature  of  which  is  not  well 
known,  but  is  called  water  of  crystallization.  On  the  aver- 
age, it  seems  likely  that  the  materials  of  the  earth,  whether 
under  the  sea  or  on  the  land,  have  several  per  cent  of  their 
mass  of  the  fluid. 

It  is  not  yet  known  to  what  depth  the  water-bearing 
section  of  the  earth  extends;  but,  as  we  shall  see  more  par- 
ticularly hereafter  when  we  come  to  consider  volcanoes, 
the  lavas  which  they  send  up  to  the  surface  are  full  of 
contained  water,  which  passes  from  them  in  the  form  of 
steam.  The  very  high  temperature  of  these  volcanic  ejec- 
tions makes  it  necessary  for  us  to  suppose  that  they  come 
from  a  great  depth.  It  is  difficult  to  believe  that  they 
originate  at  less  than  a  hundred  miles  below  the  earth's 
surface.  If,  then,  the  rocks  contain  an  average  of  even 
five  per  cent  of  water  to  the  depth  of  one  hundred  miles, 
the  quantity  of  the  fluid  stored  within  the  earth  is  greater 
than  that  which  is  contained  in  the  reservoir  of  the  ocean. 


126  OUTLINES  OF  THE  EARTH'S  HISTORY. 

The  oceans,  on  the  average,  are  not  more  than  three  miles 
deep;  spread  evenly  over  the  surface  of  the  whole  earth, 
their  depth  would  be  less  than  two  miles,  while  the  water 
in  the  rocks,  if  it  could  be  added  to  the  seas,  would  make 
the  total  depth  seven  miles  or  more.  As  we  shall  note 
hereafter,  the  processes  of  formation  of  strata  tend  to  im- 
prison water  in  the  beds,  which  in  time  is  returned  to  the 
earth's  surface  by  the  forces  which  operate  within  the 
crust. 

Although  the  water  in  the  seas  is,  as  we  have  seen, 
probably  less  than  one  half  of  the  store  which  the  earth 
possesses,  the  part  it  plays  in  the  economy  of  the  planet 
is  in  the  highest  measure  important.  The  underground 
water  operates  solely  to  promote  certain  changes  which 
take  place  in  the  mineral  realm.  Its  effect,  except  in  vol- 
canic processes,  are  brought  about  but  slowly,  and  are  lim- 
ited in  their  action.  The  movements  of  this  buried  water 
are  exceedingly  gradual;  the  forces  which  impel  it  about 
and  which  bring  it  to  do  its  work  originate  in  the  earth. 
In  the  seas  the  fluid  has  an  exceeding  freedom  of  motion; 
it  can  obey  the  varied  impulses  which  the  solar  energy  im- 
poses upon  it.  The  role  of  these  wonderful  actions  which 
we  are  about  to  trace  includes  almost  everything  which  goes 
on  upon  the  surface  of  the  planet — that  which  relates  to 
the  development  of  animal  and  vegetable  life,  as  well  as  to 
the  vast  geological  changes  which  the  earth  is  undergoing. 

If  the  surface  of  the  earth  were  uniformly  covered 
with  water  to  the  depth  of  ten  thousand  feet  or  more,  every 
particle  of  fluid  would,  in  a  measure,  obey  the  attraction 
of  the  sun,  of  the  moon,  and,  theoretically,  also  of  all  the 
other  bodies  in  space,  on  the  principle  that  every  particle 
of  matter  in  the  universe,  exercises  a  gravitative  effect  on 
every  other.  As  it  is,  owing  to  the  divided  condition  of 
the  water  on  the  earth's  surface,  only  that  which  is  in  the 
ocean  and  larger  seas  exhibits  any  measurable  influence 
from  these  distant  attractions.  In  fact,  only  the  tides  pro- 
duced by  the  moon  and  sun  are  of  determinable  magnitude. 


THE  ATMOSPHERE.  127 

and  of  these  the  lunar  is  of  greater  importance,  the  reason 
being  the  near  position  of  our  satellite  to  our  own  sphere. 
The  solar  tide  is  four  tenths  as  great  as  the  lunar.  The 
water  doubtless  obeys  in  a  slight  way  the  attraction  of 
the  other  celestial  bodies,  but  the  motions  thus  imparted 
are  too  small  to  be  discerned;  they  are  lost  in  the  great 
variety  of  influences  which  affect  all  the  matter  on  the 
earth. 

Although  the  tides  are  due  to  the  attraction  of  the 
solar  bodies,  mainly  to  that  of  the  moon,  the  mode  in 
which  the  result  is  brought  about  is  somewhat  complicated. 
It  may  briefly  and  somewhat  incompletely  be  stated  as  fol- 
lows: Owing  to  the  fact  that  the  attracting  power  of 
the  earth  is  about  eighty  times  greater  than  that  of  the 
moon,  the  centre  of  gravity  of  the  two  bodies  lies  within 
the  earth.  About  this  centre  the  spheres  revolve,  each  in  a 
way  swinging  around  the  other.  At  this  point  there  is  no 
centrifugal  motion  arising  from  the  revolution  of  the  pair 
of  spheres,  but  on  the  side  of  the  earth  opposite  the  moon, 
some  six  thousand  miles  away,  the  centrifugal  force  is  con- 
siderable, becoming  constantly  greater  as  we  pass  away 
from  the  turning  point.  At  the  same  time  the  attraction 
of  the  moon  on  the  water  becomes  less.  Thus  the  tide  op- 
posite the  satellite  is  formed.  On  the  side  toward  the  moon 
the  same  centrifugal  action  operates,  though  less  effectively 
than  in  the  other  case,  for  the  reason  that  the  turning  point 
is  nearer  the  surface;  but  this  action  is  re-enforced  by  the 
greater  attraction  of  the  moon,  due  to  the  fact  that  the 
water  is  much  nearer  that  body. 

In  the  existing  conditions  of  the  earth,  what  we  may 
call  the  normal  run  of  the  tides  is  greatly  interrupted. 
Only  in  the  southern  ocean  can  the  waters  obey  the  lunar 
and  solar  attraction  in  anything  like  a  normal  way.  In 
that  part  of  the  earth  two  sets  of  tides  are  discernible, 
the  one  and  greater  due  to  the  moon,  the  other,  much  ' 
smaller,  to  the  sun.  As  these  tides  travel  round  at  differ- 
ent rates,  the  movements  which  they  produce  are  some- 


X28         OUTLINES  OF  THE  EARTH'S  HISTORY. 

times  added  to  each  other  and  sometimes  subtracted — that 
is,  at  times  they  come  together,  while  again  the  elevation 
of  one  falls  in  the  hollow  of  the  other.  Once  again  sup- 
posing the  earth  to  be  all  ocean  covered,  computation 
shows  that  the  tides  in  such  a  sea  would  be  very  broad 
waves,  having,  indeed,  a  diameter  of  half  the  earth's  cir- 
cumference. Those  produced  by  the  moon  would  have 
an  altitude  of  about  one  foot,  and  those  by  the  sun  of  about 
three  inches.  The  geological  effects  of  these  swayings 
would  be  very  slight;  the  water  would  pass  over  the  bot- 
tom to  and  fro  twice  each  day,  with  a  maximum  journey 
of  a  hundred  or  two  feet  each  way  from  a  fixed  point. 
This  movement  would  be  so  slow  that  it  could  not  stir 
the  fine  sediment;  its  only  influence  would  perhaps  be  to 
help  feed  the  animals  which  were  fixed  upon  the  bottom 
by  drawing  the  nurture-bringing  water  by  their  mouths. 
Although  the  divided  condition  of  the  ocean  perturbs 
the  action  of  the  tides,  so  that  except  by  chance  their  waves 
are  rarely  with  their  centres  where  the  attracting  bodies 
tend  to  make  them,  the  influence  of  these  divisions  is 
greatly  to  increase  the  geological  or  change-bringing  in- 
fluences arising  from  these  movements.  When  from  the 
southern  ocean  the  tides  start  to  the  northward  up  the 
bays  of  the  Atlantic,  the  Pacific,  or  the  Indian  Ocean, 
they  have,  as  before  noted,  a  height  of  perhaps  less  than 
two  feet.  As  they  pass  up  the  narrowing  spaces  the  waves 
become  compressed — that  is,  an  equal  volume  of  moving 
water  has  less  horizontal  room  for  its  passage,  and  is  forced 
to  rise  higher.  We  see  a  tolerably  good  illustration  of  the 
same  principle  when  we  observe  a  wind-made  wave  enter  a 
small  recess  of  the  shore,  the  sides  of  which  converge  in 
the  direction  of  the  motion.  With  the  diminished  room, 
the  wave  gains  in  height.  It  thus  comes  about  that  the 
tide  throughout  the  Atlantic  basin  is  much  higher  than 
in  the  southern  ocean.  On  the  same  principle,  when  the 
tide  rolls  in  against  the  shores  every  embayment  of  a 
distinct  kind,  whose  sides  converge  toward  the  head,  packs 


The  atmosphere.  120 

up  the  tidal  wave,  often  increasing  its  height  in  a  remark- 
able way.  When  these  bays  are  wide-mouthed  and  of  elon- 
gate triangular  form,  with  deep  bottoms,  the  tides  which 
on  their  outer  parts  have  a  height  of  ten  or  fifteen  feet 
may  attain  an  altitude  of  forty  or  fifty  feet  at  the  apex  of 
the  triangle. 

We  have  already  noted  the  fact  that  the  tide,  such  as 
runs  in  the  southern  ocean,  exercises  little  or  no  influence 
upon  the  bottom  of  the  sea  over  which  it  moves.  As  the 
height  of  the  confined  waters  increases,  the  range  of  their 
journey  over  the  bottom  as  the  wave  comes  and  goes 
rapidly  increases.  When  they  have  an  elevation  of  ten 
feet  they  can  probably  stir  the  finer  mud  on  the  ocean 
floor,  and  in  shallow  water  move  yet  heavier  particles.  In 
the  embayments  of  the  land,  where  a  great  body  of  water 
journeys  like  an  alternating  river  into  extensive  basins, 
the  tidal  action  becomes  intense;  the  current  may  be  able 
to  sweep  along  large  stones  quite  as  effectively  as  a  moun- 
tain torrent.  Thus  near  Eastport,  Me.,  where  the  tides 
have  a  maximum  rise  and  fall  of  over  twenty  feet,  the 
waters  rush  in  places  so  swiftly  that  at  certain  stages  of 
the  movement  they  are  as  much  troubled  as  those  at  the 
rapids  of  the  St.  Lawrence.  In  such  portions  of  the  shore 
the  tides  do  important  work  in  carving  channels  into  the 
lands. 

Along  the  shores  of  the  continents  about  the  North 
Atlantic,  where  the  tides  act  in  a  vigorous  manner,  we 
almost  everywhere  find  an  underwater  shelf  extending 
from  the  shore  with  a  declivity  of  only  five  to  ten  feet  to 
the  mile  toward  the  centre  of  the  sea,  until  the  depth  of 
about  five  hundred  feet  is  attained;  from  this  point  the 
bottom  descends  more  steeply  into  the  ocean's  depth.  It 
is  probable  that  the  larger  part  of  the  material  composing 
these  continental  shelves  has  been  brought  to  its  position 
by  tidal  action.  Each  time  the  tidal  wave  sweeps  in  to- 
ward the  shore  it  urges  the  finer  particles  of  sediment 
along  with  it.    When  it  moves  out  it  drags  them  on  the 


130  OUTLINES  OF  THE  EARTH'S  HISTORY. 

return  journey  toward  the  depths  of  the  sea.  If  this  shelf 
were  perfectly  horizontal,  the  two  journeys  of  the  sand 
and  mud  grains  would  be  of  the  same  length;  but  as  the 
movement  takes  place  up  and  down  a  slope,  the  bits  will 
travel  farther  under  the  impulse  which  leads  them  down- 
ward than  under  that  which  impels  them  up.  The  result 
will  be  that  the  particles  will  travel  a  little  farther  out 
from  the  shore  each  time  it  is  swung  to  and  fro  in  the 
alternating  movement  of  the  tide. 

The  effect  of  tidal  movement  in  nurturing  marine  life 
is  very  great.  It  aids  the  animals  fixed  on  the  bottoms 
of  the  deep  seas  to  obtain  their  provision  of  food  and  their 
share  of  oxygen  by  drawing  the  water  by  their  bodies.  All 
regions  which  are  visited  by  strong  tides  commonly  have 
in  the  shallows  near  the  shores  a  thick  growth  of  seaweed 
which  furnishes  an  ample  provision  of  food  for  the  fishes 
and  other  forms  of  animal  life. 

A  peculiar  effect  arising  from  tidal  action  is  believed 
by  students  of  the  phenomena  to  be  found  in  the  slowing 
of  the  earth's  rotation  on  its  axis.  The  tides  rotate  around 
the  earth  from  east  to  west,  or  rather,  we  should  say,  the 
solid  mass  of  the  earth  rubs  against  them  as  it  spins  from 
west  to  east.  As  they  move  over  the  bottom  and  as  they 
strike  against  the  shores  this  push  of  the  great  waves  tends 
in  a  slight  measure  to  use  up  the  original  spinning  im- 
pulse which  causes  the  earth's  rotation.  Computation 
shows  that  the  amount  of  this  action  should  be  great 
enough  gradually  to  lengthen  the  day,  or  the  time  occu- 
pied by  the  earth  in  making  a  complete  revolution  on 
the  polar  axis.  The  effect  ought  to  be  great  enough  to 
be  measurable  by  astronomers  in  the  course  of  a  thousand 
years.  On  the  other  hand,  the  records  of  ancient  eclipses 
appear  pretty  clearly  to  show  that  the  length  of  the  day 
has  not  changed  by  as  much  as  a  second  in  the  course  of 
three  thousand  years.  This  evidence  does  not  require  us 
to  abandon  the  supposition  that  the  tides  tend  to  diminish 
the  earth's  rate  of  rotation.     It  is  more  likely  that  the 


THE  ATMOSPHERE.  131 

effect  of  the  reduction  in  the  earth's  diameter  due  to  the 
loss  of  heat  which  is  continually  going  on  counterbalances 
the  influence  of  the  tidal  friction.  As  the  diameter  of  a 
rotating  body  diminishes,  the  tendency  is  for  the  mass 
to  spin  more  rapidly;  if  it  expands,  to  turn  more  slowly, 
provided  in  each  case  the  amount  of  the  impulse  which 
leads  to  the  turning  remains  the  same.  This  can  be  di- 
rectly observed  by  whirling  a  small  weight  attached  to  a 
string  in  such  a  manner  that  the  cord  winds  around  the 
finger  with  each  revolution;  it  will  be  noted  that  as  the 
line  shortens  the  revolution  is  more  quickly  accomplished. 
We  can  readily  conceive  that  the  earth  is  made  up  of 
weights  essentially  like  that  used  in  the  experiment,  each 
being  drawn  toward  the  centre  by  the  gravitative  stress, 
which  is  like  that  applied  to  the  weight  by  the  cord. 

The  fact  that  the  days  remain  of  the  same  length 
through  vast  periods  of  time  is  probably  due  to  this  bal- 
ance between  the  effects  of  tidal  action  and  those  arising 
from  the  loss  of  heat — in  other  words,  we  have  here  one  of 
those  delicate  arrangements  in  the  way  of  counterpoise 
which  serve  to  maintain  the  balanced  conditions  of  the 
earth's  surface  amid  the  great  conflicts  of  diverse  energies 
which  are  at  work  in  and  upon  the  sphere. 

It  should  be  understood  that  the  effects  of  the  attrac- 
tion which  produces  tides  are  much  more  extensive  than 
they  are  seen  to  be  in  the  movements  of  the  sea.  So  long 
as  the  solar  and  planetary  spheres  remain  fluid,  the  whole 
of  their  masses  partake  of  the  movement.  It  is  a  conse- 
quence of  this  action,  as  the  computations  of  Prof.  George 
Darwin  has  shown,  that  the  moon,  once  nearer  the  earth 
than  it  is  at  present,  has  by  a  curious  action  of  the  tidal 
force  been  pushed  away  from  the  centre  of  our  sphere,  or 
rather  the  two  bodies  have  repelled  each  other.  An  Ameri- 
can student  of  the  problem,  Mr.  T.  J.  J.  See,  has  shown 
that  the  same  action  has  served  to  give  to  the  double  stars 
the  exceeding  eccentricity  of  their  orbits. 

Although  these  recent  studies  of  tidal  action  in  the 


132  OUTLINES  OF  THE  EARTH'S  HISTORY. 

celestial  sphere  are  of  the  utmost  importance  to  the  theory 
of  the  universe,  for  they  may  lead  to  changes  in  the  nebular 
hypotheses,  they  are  as  yet  too  incomplete  and  are,  more- 
over, too  mathematical  to  be  presented  in  an  elementary 
treatise  such  as  this. 

We  now  turn  to  another  class  of  waves  which  are  of 
even  more  importance  than  those  of  the  tides — to  the  un- 
dulations which  are  produced  by  the  action  of  the  wind  on 
the  surface  of  the  Avater.  While  the  tide  waves  are  limited 
to  the  open  ocean,  and  to  the  seas  and  bays  which  afford 
them  free  entrance,  wind  waves  are  produced  everywhere 
where  water  is  subjected  to  the  friction  of  air  which  flows 
over  it.  While  tidal  waves  come  upon  the  shores  but  twice 
each  day,  the  wind  waves  of  ordinary  size  which  roll  in 
from  the  ocean  deliver  their  blows  at  intervals  of  from  three 
to  ten  seconds.  Although  the  tidal  waves  sometimes,  by 
a  packing-up  process,  attain  the  height  of  fifty  feet,  their 
average  altitude  where  they  come  in  contact  with  the  shore 
probably  does  not  much  exceed  four  feet;  usually  they 
come  in  gently.  It  is  likely  that  in  a  general  way  the  ocean 
surges  which  beat  against  the  coast  are  of  greater  altitude. 

Wind  waves  are  produced  and  perform  their  work  in 
a  manner  which  we  shall  now  describe.  When  the  air 
blows  over  any  resisting  surface,  it  tends,  in  a  way  which 
we  can  hardly  afford  here  to  describe,  to  produce  motions. 
If  the  particle  is  free  to  move  under  the  impulse  which  it 
communicates,  it  bears  it  along;  if  it  is  linked  together  in 
the  manner  of  large  masses,  which  the  wind  can  not  trans- 
port, it  tends  to  set  it  in  motion  in  an  alternating  way. 
The  sounds  of  our  musical  instruments  which  act  by  wind 
are  due  to  these  alternating  vibrations,  such  as  all  air  cur- 
rents tend  to  produce.  An  ^olian  harp  illustrates  the 
action  which  we  are  considering.  Moving  over  matter 
which  has  the  qualities  that  we  denote  by  the  term  fluid, 
the  swayings  which  the  air  produces  are  of  a  peculiar  sort, 
though  they  much  resemble  those  of  the  fiddle  string. 


THE  ATMOSPHERE.  133 

The  surface  of  the  liquid  rises  and  falls  in  what  we  term 
waves,  the  size  of  which  is  determined  by  the  measure  of 
fluidity,  and  by  the  energy  of  the  wind.  Thus,  because  fresh 
water  is  considerably  lighter  than  salt,  a  given  wind  will 
produce  in  a  given  distance  for  the  run  of  the  waves 
heavier  surges  in  a  lake  than  it  will  in  the  sea.  For  this 
reason  the  surges  in  a  great  storm  which  roll  on  the  ocean 
shore,  because  of  the  wide  water  over  which  they  have 
gathered  their  impetus,  are  in  size  very  much  greater  than 
those  of  the  largest  lakes,  which  do  not  afford  room  for 
the  development  of  great  undulations. 

To  the  eye,  a  wave  in  the  water  appears  to  indicate  that 
the  fluid  is  borne  on  before  the  wind.  Examination,  how- 
ever, shows  that  the  amount  of  motion  in  the  direction  in 
which  the  wind  is  blowing  is  very  slight.  We  may  say, 
indeed,  that  the  essential  feature  of  a  wave  is  found  in  the 
transmission  of  impulse  rather  than  in  the  movement  of 
the  fluid  matter.  A  strip  of  carpet  when  shaken  sends 
through  its  length  undulations  which  are  almost  exactly 
like  water  waves.  If  we  imagine  ourselves  placed  in  a 
particle  of  water,  moving  in  the  swayings  of  a  wave  in  the 
open  and  deep  sea,  we  may  conceive  ourselves  carried 
around  in  an  ellipse,  in  each  revolution  returning  through 
nearly  the  same  orbit.  Now  and  then,  when  the  particle 
came  to  the  surface,  it  would  experience  the  slight  drift 
which  the  continual  friction  of  the  wind  imposes  on  the 
water.  If  the  wave  in  which  the  journey  was  made  lay 
in  the  trade  winds,  where  the  long-continued,  steadfast 
blowing  had  set  the  water  in  motion  to  great  depths,  the 
orbit  traversed  would  be  moving  forward  with  some  rapid- 
ity; where  also  the  wind  was  strong  enough  to  blow  the 
tops  of  the  waves  over,  forming  white-caps,  the  advance  of 
the  particle  very  near  the  surface  would  be  speedy.  Not- 
withstanding these  corrections,  waves  are  to  be  regarded 
each  as  a  store  of  energy,  urging  the  water  to  sway  much 
in  the  manner  of  a  carpet  strip,  and  by  the  swaying  con- 
veying the  energy  in  the  direction  of  the  wave  movement. 


134  OUTLINES  OP  THE  EARTH'S  HISTORY. 

The  rate  of  movement  of  wind  waves  increases  with 
their  height.  Slight  undulations  go  forward  at  the  rate  of 
less  than  half  a  mile  an  hour.  The  greater  surges  of  the 
deeps  when  swept  by  the  strongest  winds  move  with  the 
speed  which,  though  not' accurately  determined,  has  been 
estimated  by  the  present  writer  as  exceeding  forty  miles 
an  hour.  As  these  surges  often  have  a  length  transverse 
to  the  wind  of  a  mile  or  more,  a  width  of  about  an  eighth 
of  a  mile,  and  a  height  of  from  thirty-five  to  forty-five 
feet,  the  amount  of  energy  which  they  transmit  is  very 
great.  If  it  could  be  effectively  applied  to  the  shores  in 
the  manner  in  which  the  energy  of  exploding  gunpowder 
is  applied  by  cannon  shot,  it  is  doubtful  whether  the  lands 
could  have  maintained  their  position  against  the  assaults 
of  the  sea.  But  there  are  reasons  stated  below  why  the 
ocean  waves  can  use  only  a  very  small  part  of  their  energy 
in  rending  the  rocks  against  which  they  strike  on  the  coast 
line. 

In  the  first  place,  we  should  note  that  wind  waves 
have  very  little  influence  on  the  bottom  of  the  deep  sea. 
If  an  observer  could  stand  on  the  sea  floor  at  the  depth 
of  a  mile  below  a  point  over  which  the  greatest  waves  were 
rolling,  he  could  not  with  his  unaided  senses  discern  that 
the  water  was  troubled.  He  would,  indeed,  require  in- 
struments of  some  delicacy  to  find  out  that  it  moved  at  all. 
Making  the  same  observations  at  the  depth  of  a  thousand 
feet,  it  is  possible  that  he  would  note  a  slight  swaying 
motion  in  the  water,  enough  sensibly  to  affect  his  body. 
At  five  hundred  feet  in  depth  the  movement  would  prob- 
ably be  sufficient  to  disturb  fine  mud.  At  two  hundred 
feet,  the  rasping  of  the  surge  on  the  bottom  would  doubt- 
less be  sufficient  to  push  particles  of  coarse  sand  to  and 
fro.  At  one  hundred  feet  in  depth,  the  passage  of  the 
surge  would  be  strong  enough  to  urge  considerable  pebbles 
before  it.  Thence  up  the  slope  the  driving  action  would 
become  more  and  more  intense  until  we  attained  the  point 
where  the  wave  broke.     It  should  furthermore  be  noted 


THE  ATMOSPHERE.  135 

that,  while  the  movement  of  the  water  on  the  floor  of  the 
deep  sea  as  the  wave  passes  overhead  would  be  to  and  fro, 
with  every  advance  in  the  shallowing  and  consequent  in- 
creased friction  on  the  bottom,  the  forward  element  in  the 
movement  would  rapidly  increase.  Near  the  coast  line 
the  effect  of  the  waves  is  continually  to  shove  the  detritus 
up  the  slopes  of  the  continental  shelf.  Here  we  should 
note  the  fact  that  on  this  shelf  the  waves  play  a  part  ex- 
actly the  opposite  of  that  effected  by  the  tides.  The  tides, 
as  we  have  noted,  tend  to  drag  the  particles  down  the  slope, 
while  the  waves  operate  to  roll  them  up  the  declivity. 

As  the  wave  in  advancing  toward  the  shore  ordinarily 
comes  into  continually  shallowing  water,  the  friction  on 
the  bottom  is  ever-increasing,  and  serves  to  diminish  the 
energy  the  surge  contains,  and  therefore  to  reduce  its  pro- 
portions. If  this  action  operated  alone,  the  subtraction 
which  the  friction  makes  would  cause  the  surf  waves  which 
roll  in  over  a  continental  shelf  to  be  very  low,  probably 
in  height  less  than  half  that  which  they  now  attain.  In 
fact,  however,  there  is  an  influence  at  work  to  increase  the 
height  of  the  waves  at  the  expense  of  its  width.  Noting 
that  the  friction  rapidly  increases  with  the  shallowing,  it 
is  easy  to  see  that  this  resistance  is  greatest  on  the  advan- 
cing front  of  the  wave,  and  least  on  its  seaward  side.  The 
result  is  that  the  front  moves  more  slowly  than  the  rear, 
so  that  the  wave  is  forced  to  gain  in  height;  but  for  the 
fact  that  the  total  friction  which  the  wave  encounters 
takes  away  most  of  its  impetus,  we  might  have  combers 
a  hundred  feet  high  rolling  upon  the  shelving  shores  which 
almost  everywhere  face  the  seas. 

As  the  wave  shortens  its  width  and  gains  in  relative 
height,  though  not  in  actual  elevation,  another  action  is 
introduced  which  has  momentous  consequences.  The  water 
in  the  bottom  of  the  wave  is  greatly  retarded  in  its  ongoing 
by  its  friction  over  the  sea  floor,  while  the  upper  part  of 
the  surge  is  much  less  affected  in  this  way.  The  result  is 
that  at  a  certain  point  in  the  advance,  the  place  of  which 
10 


136  OUTLINES  OF  THE  EARTH'S  HISTORY. 

is  determined  by  the  depths  the  size^  and  the  speed  of  the 
undulation,  the  front  swiftly  steepens  until  it  is  vertical, 
and  the  top  shoots  forward  to  a  point  where  it  is  no  longer 
supported  by  underlying  water,  when  it  plunges  down  in 
what  is  called  the  surf  or  breaker.  In  this  part  of  the 
wave's  work  the  application  of  the  energy  w^hich  it  trans- 
mits differs  strikingly  from  the  w^ork  previously  done.  Be- 
fore the  wave  breaks,  the  only  geological  task  which  it 
accomplishes  is  effected  by  forcing  materials  up  the  slope, 
in  which  movement  they  are  slightly  ground  over  each 
other  until  they  come  within  the  battering  zone  of  the 
shore,  where  they  may  be  further  divided  by  the  action  of 
the  mill  which  is  there  in  operation. 

When  the  wave  breaks  on  the  shore  it  operates  in  the 
following  manner:  First,  the  overturning  of  its  crest  sends 
a  great  mass  of  w^ater,  it  may  be  from  the  height  of  ten 
or  more  feet,  down  upon  the  shore.  Thus  falling  water 
has  not  only  the  force  due  to  its  drop  from  the  summit  of 
the  wave,  but  it  has  a  share  of  the  impulse  due  to  the 
velocity  with  w^hich  the  surge  moved  against  the  shore. 
It  acts,  in  a  word,  like  a  hammer  swung  down  by  a  strong 
arm,  where  the  blow  represents  not  only  the  force  with 
which  the  weight  would  fall  of  itself,  but  the  impelling 
power  of  the  man's  muscles.  Any  one  who  will  expose  his 
body  to  this  blow  of  the  surf  will  recognise  how  violent 
it  is;  he  may,  if  the  beach  be  pebbly,  note  how  it  drives  the 
stones  about;  fragments  the  size  of  a  man's  head  may 
be  hurled  by  the  stroke  to  the  distance  of  twenty  feet 
or  more;  those  as  large  aS  the  fist  may  be  thrown  clear 
beyond  the  limits  of  the  wave.  So  vigorous  is  this  stroke 
that  the  sound  of  it  may  sometimes  be  heard  ten  miles 
inland  from  the  coast  where  it  is  delivered. 

Moving  forward  up  the  slope  of  a  gently  inclined 
beach,  the  fragments  of  the  wave  are  likely  to  be  of  suffi- 
cient volume  to  permit  them  to  regather  into  a  secondary 
surge,  which,  like  the  first,  though  much  smaller,  again 
rises  into  a  wall,  forming  another  breaker.    Under  favour- 


THE  ATMOSPHERE.  137 

able  conditions  as  many  as  four  or  five  of  these  successive 
diminishing  surf  lines  may  be  seen.  The  present  writer 
has  seen  in  certain  cases  as  many  as  a  dozen  in  the  great 
procession,  the  lowest  and  innermost  only  a  few  inches 
high,  the  outer  of  all  with  a  height  of  perhaps  twenty  feet. 

Along  with  the  direct  bearing  action  of  the  surf  goes 
a  to-and-fro  movement,  due  to  the  rushing  up  and  down 
of  the  water  on  the  beach.  This  swashing  affects  not  only 
the  broken  part  of  the  waves,  but  all  the  water  between 
the  outer  breaker  and  the  shore.  These  swayings  in  the 
surf  belt  often  swing  the  debris  on  the  inner  margin  over  a 
range  of  a  hundred  feet  or  more,  the  movement  taking 
place  with  great  swiftness,  affecting  the  pebbles  to  the 
depth  of  several  inches,  and  grinding  the  bits  together  in 
a  violent  way.  Listening  to  the  turmoil  of  a  storm,  we 
can  on  a  pebbly  beach  distinctly  hear  the  sound  of  the 
downward  stroke,  a  crashing  tone,  and  the  roar  of  the 
rolling  stones. 

As  waves  are  among  the  interesting  things  in  the  world, 
partly  on  account  of  their  living  quality  and  partly  be- 
cause of  their  immediate  and  often  exceeding  interest  to 
man,  we  may  here  note  one  or  two  peculiar  features  in 
their  action.  In  the  first  place,  as  the  reader  who  has 
gained  a  sense  of  the  changes  in  form  of  action  may  readily 
perceive,  the  beating  of  waves  on  the  shore  converts  the 
energy  which  they  possess  into  heat.  This  probably  warms 
the  water  during  great  storms,  so  that  by  the  hand  we  may 
note  the  difference  in  temperature  next  the  coast  line  and 
in  the  open  waters.  This  relative  warmth  of  the  surf 
water  is  perhaps  a  matter  of  some  importance  in  limiting 
the  development  of  ice  along  the  shore  line;  it  may  also 
favour  the  protection  of  the  coast  life  against  the  severe 
cold  of  the  winter  season. 

The  waves  which  successively  come  against  the  shore 
in  any  given  time,  particularly  if  a  violent  wind  is  blowing 
on  to  the  coast,  are  usually  of  about  the  same  size.  When, 
however,  in  times  of  calm  an  old  sea,  as  it  is  called,  is  roll- 


138  OUTLINES  OF  THE  EARTH'S  HISTORY.     . 

ing  in,  the  surges  may  occasionally  undergo  very  great 
variations  in  magnitude.  Not  infrequently  these  occa- 
sional waves  are  great  enough  to  overwhelm  persons  who 
are  upon  the  rocks  next  the  shore.  These  greater  surges 
are  probably  to  be  accounted  for  by  the  fact  that  in  the 
open  sea  waves  produced  by  winds  blowing  in  different 
directions  may  run  on  with  their  diverse  courses  and  varied 
intervals  until  they  come  near  the  shore.  Running  in  to- 
gether, it  very  well  happens  that  two  of  the  surges  be- 
longing to  different  sets  may  combine  their  forces,  thus 
doubling  the  swell.  The  danger  which  these  conjoined 
waves  bring  is  obviously  greatest  on  cliff  shores,  where, 
on  account  of  the  depth  of  water,  the  waves  do  not  break 
until  they  strike  the  steep. 

Having  considered  in  a  general  way  the  action  of  waves 
as  they  roll  in  to  the  shore,  bearing  with  them  the  solar 
energy  which  was  contributed  to  them  by  the  winds,  we 
shall  now  take  up  in  some  detail  the  work  which  goes  on 
along  the  coast  line — work  which  is  mainly  accomplished 
by  wave  action. 

On  most  coast  lines  the  observer  readily  notes  that 
the  shore  is  divided  into  two  different  kinds  of  faces — 
those  where  the  inner  margin  of  the  wave-swept  belt  comes 
against  rocky  steeps,  and  those  bordered  by  a  strand  alto- 
gether composed  of  materials  which  the  surges  have 
thrown  up.  These  may  be  termed  for  convenience  cliff 
shores  and  wall-beach  shores.  We  shall  begin  our  inquiry 
with  cliff  shores,  for  in  those  sections  of  the  coast  line  the 
sea  is  doing  its  most  characteristic  and  important  work 
of  assaulting  the  land.  If  the  student  has  an  opportunity 
to  approach  a  set  of  cliffs  of  hard  rock  in  time  of  heavy 
storm,  when  the  waves  have  somewhere  their  maximum 
height,  he  should  seek  some  headland  which  may  offer 
him  safe  foothold  whence  he  can  behold  the  movements 
which  are  taking  place.  If  he  is  so  fortunate  as  to  have 
in  vieW;  as  well  may  be  the  case,  cliffs  which  extend  down 


^HE  ATMOSPHERE.  l-{) 

into  deep  water,  and  others  which  are  bordered  by  rude 
and  generally  steeply  sloping  beaches  covered  with  large 
stones,  he  may  perceive  that  the  waves  come  in  against 
the  cliffs  which  plunge  into  deep  water  without  taking 
on  the  breaker  form.  In  this  case  the  undulation  strikes 
but  a  moderate  blow;  the  wave  is  not  greatly  broken.  The 
part  next  the  rock  may  shoot  up  as  a  thin  sheet  to  a  con- 
siderable height;  it  is  evident  that  while  the  ongoing 
wave  applies  a  good  deal  of  pressure  to  the  steep,  it  does 
not  deliver  its  energy  in  the  effective  form  of  a  blow  as 
when  the  wave  overturns,  or  in  the  consequent  rush  of 
the  water  up  a  beach  slope.  It  is  easy  to  perceive  that  firm- 
set  rock  cliffs,  with  no  beaches  at  their  bases,  can  almost 
indefinitely  withstand  the  assaults.  On  the  steep  and 
stony  beach,  because  of  its  relatively  great  declivity,  the 
breaker  or  surf  forms  far  in,  and  even  in  its  first  plunge 
often  attains  the  base  of  the  precipice.  The  blow  of  the 
overfalling  as  well  as  that  of  the  inrush  moves  about  stones 
of  great  size;  those  three  feet  or  more  in  diameter  are  often 
hurled  by  the  action  against  the  base  of  the  steep,  strik- 
ing blows,  the  sharp  note  of  which  can  often  be  heard 
above  the  general  roar  which  the  commotion  produces. 
The  needlelike  crags  forming  isles  standing  at  a  distance 
from  the  shore,  such  as  are  often  found  along  hard  rock 
coasts,  are  singularly  protected  from  the  action  of  effective 
waves.  The  surges  which  strike  against  them  are  unarmed 
with  stones,  and  the  water  at  their  bases  is  so  deep  that  it 
does  not  sway  with  the  motion  with  sufficient  energy  to 
move  them  on  the  bottom.  Where  a  cliff  is  in  this  condi- 
tion, it  may  endure  until  an  elevation  of  the  coast  line 
brings  its  base  near  the  level  of  the  sea,  or  until  the  process 
of  decay  has  detached  a  sufficient  quantity  of  stone  to  form 
a  talus  or  inclined  plane  reaching  near  to  the  water  level. 
As  before  noted,  it  is  the  presence  of  a  sloping  beach 
reaching  to  about  the  base  of  the  cliff  which  makes  it  pos- 
sible for  the  waves  to  strike  at  with  a  hammer  instead 
of  with  a  soft  hand.     Battering  at  the  base  of  the  cliff. 


140  OUTLINES  OF  THE  EARTH'S  HISTORY. 

the  surges  cut  a  crease  along  the  strip  on  which  they 
strike,  which  gradually  enters  so  far  that  the  overhanging 
rock  falls  of  its  own  weight.  The  fragments  thus  deliv- 
ered to  the  sea  are  in  turn  broken  up  and  used  as  bat- 
tering instruments  until  they  are  worn  to  pieces.  We 
may  note  that  in  a  few  months  of  heavy  weather  the  stones 
of  such  a  fall  have  all  been  reduced  to  rudely  spherical 
forms.  Observations  made  on  the  eastern  face  of  Cape 
Ann,  Mass.,  where  the  seas  are  only  moderately  heavy, 
show  that  the  storms  of  a  single  winter  reduce  the  frag- 
ments thrown  into  the  sea  from  the  granite  quarries  to 
spheroidal  shapes,  more  than  half  of  their  weight  com- 
monly being  removed  in  the  form  of  sand  and  small  peb- 
bles which  have  been  worn  from  their  surfaces. 

We  can  best  perceive  the  effect  of  battering  action 
which  the  sea  applies  to  the  cliffs  by  noting  the  points 
where,  owing  to  some  chance  features  in  the  structure  in 
the  rock,  it  has  proved  most  effective.  Where  a  joint  or 
a  dike,  or  perhaps  a  softer  layer,  if  the  rocks  be  bedded, 
causes  the  wear  to  go  on  more  rapidly,  the  waves  soon 
excavate  a  recess  in  which  the  pebbles  are  retained,  except 
in  stormy  weather,  in  an  unmoved  condition.  When  the 
surges  are  heavy,  these  stones  are  kept  in  continuous  mo- 
tion, receding  as  the  wave  goes  back,  and  rushing  forward 
with  its  impulse  until  they  strike  against  the  firm-set  rock 
at  the  end  of  the  chasm.  In  this  way  they  may  drive  in 
a  cut  having  the  length  of  a  hundred  feet  or  more  from 
the  face  of  the  precipice.  In  most  cases  the  roofs  over 
these  sea  caves  fall  in,  so  that  the  structure  is  known  as  a 
chasm.  Occasionally  these  roofs  remain,  in  which  case, 
for  the  reason  that  the  floor  of  the  cutting  inclines  up- 
ward, an  opening  is  made  to  the  surface  at  their  upper 
end,  forming  what  is  called  in  New  England  a  '^  spouting 
horn'';  from  the  inland  end  of  the  tunnel  the  spray  may 
be  thrown  far  into  the  air.  As  long  as  the  cave  is  closed 
at  this  inner  end,  and  is  not  so  high  but  that  it  may  be 
buried  beneath  a  heavy  wave,  the  inrushing  water  com- 


THE  ATMOSPHERE.  141 

presses  the  air  in  the  rear  parts  of  the  opening.  When 
the  wave  begins  to  retreat  this  air  blows  out,  sending  a 
gust  of  spray  before  it,  the  action  resembling  the  discharge 
of  a  great  gun  from  the  face  of  a  fortification.  It  often 
happens  that  two  chasms  converging  separate  a  rock  from 
the  cliff.  Then  a  lowering  of  the  coast  may  bring  the 
mass  to  the  state  of  a  columnar  island,  such  as  abound  in 
the  Hebrides  and  along  various  other  shores. 

If  a  cliff  shore  retreats  rapidly,  it  may  be  driven  back 
into  the  shore,  and  its  face  assumes  the  curve  of  a  small 
bay.  With  every  step  in  this  change  the  bottom  is  sure  to 
become  shallower,  so  that  the  waves  lose  more  and  more 
of  their  energy  in  friction  over  the  bottom.  Moreover, 
in  entering  a  bay  the  friction  which  the  waves  encounter 
in  running  along  the  sides  is  greater  than  that  which 
they  meet  in  coming  in  upon  a  headland  or  a  straight 
shore.  The  result  is,  with  the  inward  retreat  of  the  steep 
it  enters  on  conditions  which  diminish  the  effectiveness  of 
the  wave  stroke.  The  embayment  also  is  apt  to  hold  de- 
tritus, and  so  forms  in  time  a  beach  at  the  foot  of  the  cliff, 
over  which  the  waves  rarely  are  able  to  mount  with  such 
energy  as  will  enable  them  to  strike  the  wall  in  an  effective 
manner.  With  this  sketch  of  the  conditions  of  a  cliff 
shore,  we  will  now  consider  the  fate  of  the  broken-up 
rock  which  the  waves  have  produced  on  that  section  of  the 
coast  land. 

By  observation  of  sea-beaten  cliffs  the  student  read- 
ily perceives  that  a  great  amount  of  rocky  matter  has 
been  removed  from  most  cliff-faced  shores.  Not  uncom- 
monly it  can  be  shown  that  such  sea  faces  have  retreated 
for  several  miles.  The  question  now  arises.  What  becomes 
of  the  matter  which  has  been  broken  up  by  the  wave 
action?  In  some  part  the  rock,  when  pulverized  by  the 
pounding  to  which  it  is  subjected,  has  dissolved  in  the  water. 
Probably  ninety  per  cent  of  it,  however,  retains  the  visi- 
ble state,  and  has  a  fate  determined  by  the  size  of  the  frag- 
ments of  which  it  is  composed.    If  these  be  as  fine  as  mud, 


142  OUTLINES  OF  THE  EARTH'S  HISTORY. 

so  that  they  may  float  in  the  water,  they  are  readily  borne 
away  by  the  currents  which  are  always  created  along  a 
storm-swept  shore,  particularly  by  the  undertow  or  bot- 
tom outcurrent — the  "sea-puss/'  as  it  is  sometimes  called — 
that  sweeps  along  the  bottom  from  every  shore,  against 
which  the  waves  form  a  surf.  If  as  coarse  as  sand  grains,  or 
even  very  small  pebbles,  they  are  likely  to  be  drawn  out, 
rolling  over  the  bottom  to  an  indefinite  distance  from  the 
sea  margin.  The  coarser  stones,  however,  either  remain 
at  the  foot  of  the  cliff  until  they  are  beaten  to  pieces,  or 
are  driven  along  the  shore  until  they  find  some  embay- 
ment  into  which  they  enter.  The  journey  of  such  frag- 
ments may,  when  the  wind  strikes  obliquely  to  the  shore, 
continue  for  many  miles;  the  waves,  running  with  the 
wind,  drive  the  fragments  in  oscillating  journeys  up  and 
down  the  beach,  sometimes  at  the  rate  of  a  mile  or  more 
a  day.  The  effect  of  this  action  can  often  be  seen  where  a 
vessel  loaded  with  brick  or  coal  is  wrecked  on  the  coast. 
In  a  month  fragments  of  the  materials  may  be  stretched 
along  for  the  distance  of  many  miles  on  either  side  of  the 
point  where  the  cargo  came  ashore.  Entering  an  embay- 
ment  deep  enough  to  restrain  their  further  journey,  the 
fragments  of  rock  form  a  boulder  beach,  where  the  bits  roll 
to  and  fro  whenever  they  are  struck  by  heavy  surges.  The 
greater  portion  of  them  remain  in  this  mill  until  they  are 
ground  to  the  state  of  sand  and  mud.  Now  and  then  one 
of  the  fragments  is  tossed  up  beyond  the  reach  of  the  waves, 
and  is  contributed  to  the  wall  of  the  beach.  In  very  heavy 
storms  these  pebbles  which  are  thrown  inland  may  amount 
in  weight  to  many  tons  for  each  mile  of  shore. 

The  study  of  a  pebbly  beach,  drawn  from  crest  to  the 
deep  water  outside,  will  give  an  idea  as  to  the  history  of 
its  work.  On  either  horn  of  the  crescent. by  which  the 
pebbles  are  imported  into  the  pocket  we  find  the  largest 
fragments.  If  the  shore  of  the  bay  be  long,  the  innermost 
part  of  the  recess  may  show  even  only  very  small  pebbles, 
or  perhaps  only  fine  sand,  the  coarser  material  having 


THE  ATMOSPHERE.  143 

been  worn  out  in  the  journey.  On  the  bottom  of  the  bay, 
near  low  tide,  we  begin  to  find  some  sand  produced  by 
the  grinding  action.  Yet  farther  out,  below  high -tide 
mark,  there  is  commonly  a  layer  of  mud  which  represents 
the  finer  products  of  the  mill. 

Boulder  beaches  are  so  quick  in  answering  to  every 
slight  change  in  the  conditions  which  affect  them  that  they 
seem  almost  alive.  If  by  any  chance  the  supply  of  detritus 
is  increased,  they  fill  in  between  the  horns,  diminish  the 
incurve  of  the  bay,  and  so  cause  its  beach  to  be  more  ex- 
posed to  heavy  waves.  If,  on  the  other  hand,  the  supply 
of  grist  to  the  mill  is  diminished,  the  beach  becomes  more 
deeply  incurved,  and  the  wave  action  is  proportionately 
reduced.  We  may  say,  in  general,  that  the  curve  of  these 
beaches  represents  a  balance  between  the  consumption 
and  supply  of  the  pebbles  which  they  grind  up.  The  sup- 
ply of  pebbles  brought  along  the  shore  by  the  waves  is 
in  many  cases  greatly  added  to  by  a  curious  action  of  sea- 
weeds. If  the  bottom  of  the  water  off  the  coast  is  covered 
by  these  fragments,  as  is  the  case  along  many  coast  lines 
within  the  old  glaciated  districts,  the  spores  of  algae  are 
prone  to  take  root  upon  them.  Fastening  themselves  in 
those  positions,  and  growing  upward,  the  seaweeds  may 
attain  considerable  size.  Being  provided  with  floats,  the 
plant  exercises  a  certain  lifting  power  on  the  stone,  and 
finally  the  tugging  action  of  the  waves  on  the  fronds  may 
detach  the  fragments  from  the  bottom,  making  them  free 
to  journey  toward  the  shore.  Observing  from  near  at  hand 
the  straight  wall  of  the  wave  in  times  of  heavy  storm,  the 
present  writer  has  seen  in  one  view  as  many  as  a  dozen 
of  these  plant-borne  stones,  sometimes  six  inches  in  diame- 
ter, hanging  in  the  walls  of  water  as  it  was  about  to  topple 
over.  As  soon  as  they  strike  the  wave-beaten  part  of  the 
shore  these  stones  are  apt  to  become  separated  from  the 
plants,  though  we  can  often  notice  the  remains. or  prints 
of  the  attachments  adhering  to  the  surface  of  the  rock. 
AVhere  the  pebbles  off  the  shore  are  plenty,  a  rocky  beach 


144  OUTLINES  OF  THE  EARTH'S  HISTORY. 

may  be  produced  by  this  process  of  importation  through 
the  agency  of  seaweeds  without  any  supply  being  brought 
by  the  waves  along  the  coast  line. 

Eeturning  to  sand  beaches,  we  enter  the  most  inter- 
esting field  of  contact  between  seas  and  lands.  Probably 
nine  tenths  of  all  the  coast  lines  of  the  open  ocean  are 
formed  of  arenaceous  material.  In  general,  sand  con- 
sists of  finely  broken  crystals  of  silica  or  quartz.  These 
bits  are  commonly  distinctly  faceted;  they  rarely  have  a 
spherical  form.  Not  only  do  accumulations  of  sand  border 
most  of  the  shore  line,  but  they  protect  the  land  against 
the  assaults  of  the  sea,  and  this  in  the  following  curious 
manner:  When  shore  waves  beat  pebbles  against  each 
other,  they  rapidly  wear  to  bits;  we  can  hear  the  sound  of 
the  wearing  action  as  the, wave  goes  to  and  fro.  We  can 
often  see  that  the  water  is  discoloured  by  the  mud  or 
powdered  rock.  When,  however,  the  waves  tumble  on  a 
sandy  coast,  they  make  but  a  muffled  sound,  and  pro- 
duce no  mud.  In  fact,  the  particles  of  sand  do  not  touch 
each  other  when  they  receive  the  blow.  Between  them 
there  lies  a  thin  film  of  water,  drawn  in  by  the  attrac- 
tion known  as  capillarity,  which  sucks  the  fluid  into  a 
sponge  or  between  plates  of  glass  placed  near  together. 
The  stroke  of  the  waves  slightly  compresses  this  capillary 
water,  but  the  faces  of  the  grains  are  kept  apart  as  sheets 
of  glass  may  be  observed  to  be  restrained  from  contact 
when  water  is  between  them.  If  the  reader  would  con- 
vince himself  as  to  the  condition  of  the  sand  grains  and 
the  water  which  is  between  them,  he  may  do  so  by  pressing 
his  foot  on  the  wet  beach  which  the  wave  has  just  left. 
He  will  observe  that  it  whitens  and  sinks  a  little  under 
the  pressure,  but  returns  in  good  part  to  its  original  form 
when  the  foot  is  lifted.  In  the  experiment  he  has  pushed 
a  part  of  the  contained  water  aside,  but  he  has  not  brought 
the  grains  together;  they  do  not  make  the  sound  which  he 
will  often  hear  when  the  sand  is  dry.  The  result  is  that 
the  sand  on  the  seashore  may  wear  more  in  going  the  dis- 


THE  ATMOSPHERE.  145 

tance  of  a  mile  in  the  dry  sand  dune  than  in  traveUing 
for  hundreds  along  the  wet  shore. 

If  the  rock  matter  in  the  state  of  sand  wore  as  rapidly 
under  the  beating  of  the  waves  as  it  does  in  the  state  of 
pebbles,  the  continents  would  doubtless  be  much  smaller 
than  they  are.  Those  coasts  which  have  no  other  pro- 
tection than  is  afforded  by  a  low  sand  beach  are  often 
better  guarded  against  the  inroads  of  the  sea  than  the  rock- 
girt  parts  of  the  continents.  It  is  on  account  of  this  re- 
markable endurance  of  sand  of  the  action  of  the  waves 
tliat  the  stratified  rocks  which  make  up  the  crust  of  the 
earth  are  so  thick  and  are  to  such  an  extent  composed  of 
sand  grains. 

The  tendency  of  the  deh'is-makmg  influences  along 
the  coast  line  is  to  fill  in  the  irregularities  which  normally 
exist  there;  to  batter  off  the  headlands,  close  up  the  bays 
and  harbours,  and  generally  to  reduce  the  shores  to  straight 
lines.  Where  the  tide  has  access  to  these  inlets,  it  is  con- 
stantly at  work  in  dragging  out  the  detritus  which  the 
waves  make  and  thrust  into  tlie  recesses.  These  two  actions 
contend  with  each  other,  and  determine  the  conditions  of 
the  coast  line,  whether  they  afford  ports  for  commerce  or 
are  sealed  in  by  sand  bars,  as  are  many  coast  lines  which 
are  not  tide-swept,  as  that  of  northern  Africa,  which  faces 
the  Mediterranean,  a  nearly  tideless  sea.  The  same  is  the 
case  with  the  fresh-water  lakes;  even  the  greater  of  them 
are  often  singularly  destitute  of  shelters  which  can  serve 
the  use  of  ships,  and  this  because  there  are  no  tides  to  keep 
the  bays  and  harbours  open. 

The  Ocean  Currents. 

The  system  of  ocean  currents,  though  it  exhibits  much 
complication  in  detail,  is  in  the  main  and  primarily  de- 
pendent on  the  action  of  the  constant  air  streams  known 
as  the  trade  winds.  AVith  the  breath  from  the  lips  over 
a  basin  of  water  we  can  readily  make  an  experiment  which 


U6  OUTLINES  OF  THE  EARTH'S  HISTORY. 

shows  in  a  general  way  the  method  in  which  the  wdnds 
operate  in  producing  the  circulation  of  the  sea.  Blowing 
upon  the  surface  of  the  w^ater  in  the  basin,  we  find  that 
even  this  slight  impulse  at  once  sets  the  upper  part  in  mo- 
tion, the  movement  being  of  two  kinds — pulsating  move- 
ments or  waves  are  produced,  and  at  the  same  time  the 
friction  of  the  air  on  the  surface  causes  its  upper  part 
to  slide  over  the  under.  With  little  floats  we  can  shortly 
note  that  the  stream  which  forms  passes  to  the  farther 
side  of  the  vessel,  there  divides,  and  returns  to  the  point 
of  beginning,  forming  a  double  circle,  or  rather  two 
ellipses,  the  longer  sides  of  which  are  parallel  with  the  line 
of  the  air  current.  Watching  more  closely,  aiding  the 
sight  by  the  particles  which  float  at  various  distances  below 
the  surface,  we  note  the  fact  that  the  motion  which  was 
at  first  imparted  to  the  surface  gradually  extends  down- 
ward until  it  affects  the  water  to  the  depth  of  some  inches. 

In  the  trade-wind  belt  the  ocean  waters  to  the  depth 
of  some  hundreds  of  feet  acquire  a  continuous  movement 
in  the  direction  in  which  they  are  impelled  by  those  winds. 
This  motion  is  most  rapid  at  the  surface  and  near  the 
tropics.  It  diminishes  downwardly  in  the  water,  and  also 
toward  the  polar  sides  of  the  trade-wind  districts.  Thus 
the  trades  produce  in  the  sea  two  broad,  slow-moving, 
deep  currents,  flowing  in  the  northern  hemisphere  toward 
the  southwest,  and  in  the  southern  hemisphere  toward 
the  northwest.  Coming  down  upon  each  other  obliquely, 
these  broad  streams  meet  about  the  middle  of  the  tropical 
belt.  Here,  as  before  noted,  the  air  of  the  trade  wdnds 
leaves  the  surface  and  rises  upward.  The  w^aters  being 
retained  on  their  level,  form  a  current  which  moves  toward 
the  west.  If  the  earth  within  the  tropics  were  covered  by 
a  universal  sea,  the  result  of  this  movement  would  be  the 
institution  of  a  current  which,  flowing  under  the  equator, 
would  girdle  the  sphere. 

With  a  girdling  equatorial  current,  because  of  the  in- 
tense heat  of  the  tropics  and  the  extreme  cold  of  the 


THE  ATMOSPHERE.  147 

parallels  beyond  the  fortieth  degree  of  latitude,  the  earth 
would  be  essentially  uninhabitable  to  man,  and  hardly  so 
to  any  forms  of  life.  Its  surface  would  be  visited  by  fierce 
winds  induced  by  the  very  great  differences  of  tempera- 
ture which  would  then  prevail.  Owing,  however,  to  the 
barriers  which  the  continents  interpose  to  the  motions  of 
these  windward-setting  tropical  currents,  all  the  water 
which  they  bear,  when  it  strikes  the  opposing  shores,  is 
diverted  to  the  right  and  left,  as  was  the  stream  in  the 
experiment  with  the  basin  and  the  breath,  the  divided 
currents  seeking  ways  toward  high  latitudes,  conveying 
their  store  of  heat  to  the  circumpolar  lands.  So  effective 
is  this  transfer  of  temperature  that  a  very  large  part  of  the 
heat  which  enters  the  waters  in  the  tropical  region  is  taken 
out  of  that  division  of  the  earth's  surface  and  distributed 
over  the  realms  of  sea  and  land  which  lie  beyond  the  limits 
of  the  vertical  sun.  Thus  the  Gulf  Stream,  the  northern 
branch  of  the  Atlantic  tropical  current,  by  flowing  into  the 
North  Atlantic,  contributes  to  the  temperature  of  the 
region  within  the  Arctic  Circle  more  heat  than  actually 
comes  to  that  district  by  the  direct  influx  from  the  sun. 

The  above  statements  as  to  the  climatal  effect  of  the 
ocean  streams  show  us  how  important  it  is  to  obtain  a 
sufficient  conception  as  to  the  way  in  which  these  currents 
now  move  and  what  we  can  of  their  history  during  the 
geologic  ages.  This  task  can  not  yet  be  adequately  done. 
The  fields  of  the  sea  are  yet  too  imperfectly  explored  to 
afford  us  all  the  facts  required  to  make  out  the  whole 
story.  Only  in  the  case  of  our  Gulf  Stream  can  we  form 
a  full  conception  as  to  the  journey  which  the  waters  under- 
go and  the  consequence  of  their  motion.  In  the  case  of 
this  current,  observations  clearly  show  that  it  arises  from 
the  junction  near  the  equatorial  line  of  the  broad  stream 
created  by  the  two  trade-wind  belts.  Uniting  at  the  equa- 
tor, these  produce  a  westerly  setting  current,  having  the 
width  of  some  hundred  miles  and  a  depth  of  several 
hundred  feet.     Its  velocity  is  somewhat  greater  than  a 


us  OUTLINES  OF  THE  EARTH'S  HISTORY. 

mile  an  hour.  The  centre  of  the  current,  because  of  the 
greater  strength  of  the  northern  as  compared  with  the 
southern  trades,  is  considerably  south  of  the  equator. 
When  this  great  slow-moving  stream  comes  against  the 
coast  of  South  America,  it  encounters  the  projecting 
shoulder  of  that  land  which  terminates  at  Cape  St.  Roque. 
There  it  divides,  as  does  a  current  on  the  bows  of  an  an- 
chored ship,  a  part — rather  more  than  one  half — of  the 
stream  turning  to  the  northward,  the  remainder  passing 
toward  the  southern  pole;  this  northerly  portion  becomes 
what  is  afterward  known  as  the  Gulf  Stream,  the  history 
of  which  we  shall  now  briefly  follow. 
,  Flowing  by  the  northwesterly  coast  of  South  America, 
the  northern  share  of  the  tropical  current,  being  pressed 
in  against  the  land  by  the  trade  winds,  is  narrowed,  and 
therefore  acquires  at  once  a  swifter  flow,  the  increased 
speed  being  due  to  conditions  like  those  which  add  to  the 
velocity  of  the  water  flowing  through  a  hose  when  it  comes 
to  the  constriction  of  the  nozzle.  Attaining  the  line  of 
the  southeastern  or  Lesser  Antilles,  often  known  as  the 
Windward  Islands,  a  part  of  this  current  slips  through 
the  interspaces  between  these  isles  and  enters  the  Gulf  of 
Mexico.  Another  portion,  failing  to  find  sufficient  room 
through  these  passages,  skirts  the  Antilles  on  their  east- 
ern and  northern  sides,  passes  by  and  among  the  Bahama 
Islands,  there  to  rejoin  the  part  of  the  stream  which  en- 
tered the  Caribbean.  This  Caribbean  portion  of  the  tide 
spreads  widely  in  that  broad  sea,  is  constricted  again  be- 
tween Cuba  and  Yucatan,  again  expands  in  the  Gulf  of 
Mexico,  and  is  finally  poured  forth  through  the  Straits  of 
Florida  as  a  stream  having  the  width  of  forty  or  fifty  miles, 
a  depth  of  a  thousand  feet  or  more,  and  a  speed  of  from 
three  to  five  miles  an  hour,  exceeding  in  its  rate  of  flow 
the  average  of  the  greatest  rivers,  and  conveying  more 
water  than  do  all  the  land  streams  of  the  earth.  In  this 
part  of  its  course  the  deep  and  swift  stream  from  the 
Gulf  of  Mexico,  afterward  to  be  named  the  Gulf  Stream, 


THE  ATMOSPHERE.  149 

receives  the  contribution  of  slower  moving  and  shallower 
currents  which  skirted  the  Antilles  on  their  eastern  verge. 
The  conjoined  waters  then  move  northward,  veering  to- 
ward the  east,  at  first  as  a  swift  river  of  the  sea  having 
a  width  of  less  than  a  hundred  miles  and  of  great  depth; 
with  each  step  toward  the  pole  this  stream  widens,  dimin- 
ishing proportionately  in  depth;  the  speed  of  its  current 
decreases  as  the  original  impetus  is  lost,  and  the  baffling 
winds  set  its  surface  waters  to  and  fro  in  an  irregular  way. 
Where  it  passes  Cape  llatteras  it  has  already  lost  a  large 
share  of  its  momentum  and  much  of  its  heat,  and  is  greatly 
widened. 

Although  the  current  of  the  Gulf  Stream  becomes  more 
languid  as  we  go  northward,  it  for  a  very  long  time  retains 
its  distinction  from  the  waters  of  the  sea  through  which 
it  flows.  Sailing  eastward  from  the  mouth  of  the  Chesa- 
peake, the  navigator  can  often  observe  the  moment  when 
he  enters  the  waters  of  this  current.  This  is  notable  not 
only  in  the  temperature,  but  in  the  hue  of  the  sea.  North 
of  that  line  the  sharpness  of  the  parting  wall  becomes  less 
distinct,  the  stream  spreads  out  broadly  over  the  surface  of 
the  Atlantic,  yet  its  thermometric  effects  are  distinctly 
traceable  to  Iceland  and  Nova  Zembla,  and  the  tropical 
driftwood  which  it  carries  affords  the  principal  timber 
supply  of  the  inhabitants  of  the  first-named  isle.  Attain- 
ing this  circumpolar  realm,  and  finally  losing  the  impulse 
which  bore  it  on,  the  water  of  the  Gulf  Stream  partly 
returns  to  the  southward  in  a  relatively  slight  current 
which  bears  the  fluid  along  the  coast  of  Europe  until  it 
re-enters  the  system  of  tropical  winds  and  the  currents 
which  they  produce.  A  larger  portion  stagnates  in  the 
circumpolar  region,  in  time  slowly  to  return  to  the  tropical 
district  in  a  manner  afterward  to  be  described.  Although 
the  Gulf  Stream  in  the  region  north  of  Cape  Hatteras  is 
so  indistinct  that  its  presence  was  not  distinctly  recognised 
until  the  facts  were  subjected  to  the  keen  eye  of  Benjamin 
Franklin,  its  effects  in  the  way  of  climate  are  so  great  that 


150  OUTLINES  OF  THE  EARTH'S  HISTORY. 

we  must  attribute  the  fitness  of  northern  Europe  for  the 
uses  of  civilized  man  to  its  action.  But  for  the  heat  which 
this  stream  brings  to  the  realm  of  the  North  Atlantic, 
Great  Britain  would  be  as  sterile  as  Labrador,  and  the 
Scandinavian  region,  the  cradle-land  of  our  race,  as  unin- 
habitable as  the  bleakest  parts  of  Siberia. 

It  is  a  noteworthy  fact  that  when  the  equatorial  cur- 
rent divides  on  the  continents  against  which  it  flows,  the 
separate  streams,  although  they  may  follow  the  shores  for 
a  certain  distance  toward  the  poles,  soon  diverge  from 
them,  just  as  the  Gulf  Stream  passes  to  the  seaward  from 
the  eastern  coast  of  the  United  States.  The  reason  for 
this  movement  is  readily  found  in  the  same  principle  which 
explains  the  oblique  flow  of  the  trades  and  counter  trades 
in  their  passage  to  and  from  the  equatorial  belt.  The  par- 
ticle of  water  under  the  equator,  though  it  flows  to  the 
west,  has,  by  virtue  of  the  earth's  rotation,  an  eastward- 
setting  velocity  of  a  thousand  miles  an  hour.  Starting 
toward  the  poles,  the  particle  is  ever  coming  into  regions 
of  the  sea  where  the  fluid  has  a  less  easterly  movement, 
due  to  the  earth's  rotation  on  its  axis.  Consequently  the 
journeying  water  by  its  momentum  tends  to  move  off  in 
an  easterly  course.  Attaining  high  latitudes  and  losing 
its  momentum,  it  abides  in  the  realm  long  enough  to  be- 
come cooled. 

We  have  already  noted  the  fact  that  only  a  portion  of 
the  waters  sent  northward  in  the  Gulf  Stream  and  the 
other  currents  which  flow  from  the  equator  to  the  poles 
is  returned  by  the  surface  flow  which  sets  toward  the 
equator  along  the  eastern  side  of  the  basins.  The  largest 
share  of  the  tide  effects  its  return  journey  in  other  ways. 
Some  portion  of  this  remainder  sets  equatorward  in  local 
cold  streams,  such  as  that  which  pours  forth  through 
Davis  Strait  into  Baffin  Bay,  flowing  under  the  Gulf 
Stream  waters  for  an  unknown  distance  toward  the  tropics. 
There  are  several  of  these  local  as  yet  little  known  streams, 
which  doubtless  bring  about  a  certain  amount  of  circula- 


THE  ATMOSPHERE.  151 

tion  between  the  polar  regions  and  the  tropical  districts. 
Their  effect  is,  however,  probably  small  as  compared  with 
that  massive  drift  which  we  have  now  to  note. 

The  tropical  waters  when  they  attain  high  latitudes 
are  constantly  cooled,  and  are  overlaid,  by  the  warmer 
contributions  of  that  tide,  and  are  thus  brought  lower 
and  lower  in  the  sea.  When  they  start  downward  they 
have,  as  observations  show,  a  temperature  not  much  above 
the  freezing  point  of  salt  water.  They  do  not  congeal 
for  the  reason  that  the  salt  of  the  ocean  lowers  the  point 
at  which  the  water  solidifies  to  near  28°  Fahr.  The  effect 
of  this  action  is  gradually  to  press  down  the  surface  cold 
water  until  it  attains  the  very  bottom  in  all  the  circum- 
polar  regions.  At  the  same  time  this  descending  water 
drifts  along  the  bottom  of  the  ocean  troughs  toward  the 
equatorial  realm.  As  this  cold  water  is  heavier  than  that 
which  is  of  higher  temperature  and  nearer  the  surface, 
it  has  no  tendency  to  rise.  Being  below  the  disturbing 
influences  of  any  current  save  its  own,  it  does  not  tend, 
except  in  a  very  small  measure,  to  mingle  with  the  warmer 
overlying  fluid.  The  result  is  that  it  continues  its  jour- 
ney until  it  may  come  within  the  tropics  without  having 
gained  a  temperature  of  more  than  35°  Fahr.,  the  increase 
in  heat  being  due  in  small  measure  to  that  which  it  re- 
ceives from  the  earth's  interior  and  that  which  it  acquires 
from  the  overlying  warmer  water.  Attaining  the  region 
of  the  tropical  current,  this  drift  water  from  the  poles 
gradually  rises,  to  take  the  place  of  that  which  goes  pole- 
ward, becomes  warm,  and  again  starts  on  its  surface  jour- 
ney toward  the  arctic  and  antarctic  regions. 

Nothing  is  known  as  to  the  rate  of  this  bottom  drift 
from  th^  polar  districts  toward  the  equator,  but,  from  some 
computation  which  he  has  made,  the  writer  is  of  the  opinion 
that  several  centuries  is  doubtless  required  for  the  journey 
from  the  Arctic  Circle  to  the  tropics.  The  speed  of  the 
movement  probably  varies;  it  may  at  times  require  some 
thousand  years  for  its  accomplishment.  The  effect  of  the 
11 


152  OUTLINES  OP  THE  EARTH'S  HISTORY. 

bottom  drift  is  to  withdraw  from  seas  in  high  latitudes 
the  very  cold  water  which  there  forms,  and  to  convey  it 
beneath  the  seas  of  middle  latitudes  to  a  realm  where  it 
is  well  placed  for  the  reheating  process.  If  all  the  cold 
water  of  circumpolar  regions  had  to  journey  over  the  sur- 
face to  the  equator,  the  perturbing  effect  of  its  flow  on 
tlie  climates  of  various  lands  would  be  far  greater  than 
it  is  at  present.  Where  such  cold  currents  exist  the  effect 
is  to  chill  the  air  without  adding  much  to  the  rainfall; 
while  the  currents  setting  northward  not  only  warm  the 
regions  near  which  they  flow,  but  by  so  doing  send  from 
the  water  surfaces  large  quantities  of  moisture  which  fall 
as  snow  or  rain.  Thus  the  Gulf  Stream,  directly  and  in- 
directly, probably  contributes  more  than  half  the  rainfall 
about  the  Atlantic  basin.  The  lack  of  this  influence  on 
the  northern  part  of  North  America  and  Asia  causes  those 
lands  to  be  sterilized  by  cold,  although  destitute  of  perma- 
nent ice  and  snow  upon  their  surfaces. 

We  readily  perceive  that  the  effect  of  the  oceanic  cir- 
culation upon  the  temperatures  of  different  regions  is  not 
only  great  but  widely  contrasted.  By  taking  from  the 
equatorial  belt  a  large  part  of  the  heat  which  falls  within 
tliat  realm,  it  lowers  the  temperature  to  the  point  which 
makes  the  district  fit  for  the  occupancy  of  man,  perhaps, 
indeed,  tenable  to  all  the  higher  forms  of  life.  This  same 
heat  removed  to  high  latitudes  tempers  the  winter's  cold, 
and  thus  makes  a  vast  realm  inhabitable  wdiich  otherwise 
would  be  locked  in  almost  enduring  frosts.  Furthermore, 
this  distribution  of  temperatures  tends  to  reduce  the  total 
wind  energy  by  diminishing  the  trades  and  counter  trades 
which  are  due  to  the  variations  of  heat  wliich  are  encoun- 
tered in  passing  polarward  from  the  equator.  Still  further, 
but  for  this  circulation  of  water  in  the  sea,  the  oceans 
about  the  poles  would  be  frozen  to  their  very  bottom,  and 
this  vast  sheet  of  ice  might  be  extended  southward  to 
within  the  parallels  of  fifty  degrees  north  and  south  lati- 
tude, although  the  waters  under  the  equator  might  at  the 


THE  ATMOSPHERE.  153 

same  time  be  unendurably  hot  and  unfit  for  the  occupancy 
of  living  beings. 

A  large  part  of  the  difficulties  which  geologists  en- 
counter in  endeavouring  to  account  for  the  changes  of  the 
past  arise  from  the  evidences  of  great  climatal  revolutions 
which  the  earth  has  undergone.  In  some  chapters  of  the 
great  stone  book,  whose  leaves  are  the  strata  of  the  earth, 
we  find  it  plainly  written  in  the  impressions  made  by  fos- 
sils that  all  the  lands  beyond  the  equatorial  belt  have 
undergone  changes  which  can  only  be  explained  by  the 
supposition  that  the  heat  and  moisture  of  the  countries 
have  been  subjected  to  sudden  and  remarkable  changes. 
Thus  in  relatively  recent  times  thick-leaved  plants  which 
retained  their  vegetation  in  a  rather  tender  state  through- 
out the  year  have  flourished  near  to  the  poles,  while  short- 
ly afterward  an  ice  sheet,  such  as  now  covers  the  greater 
part  of  Greenland,  extended  down  to  the  line  of  the  Ohio 
River  at  Cincinnati.  Although  these  changes  of  climate 
are,  as  we  shall  hereafter  note,  probably  due  to  entangled 
causes,  we  must  look  upon  the  modifications  of  the  ocean 
streams  as  one  of  the  most  important  elements  in  the 
causation.  We  can  the  more  readily  imagine  such  changes 
to  be  due  to  the  alterations  in  the  course  and  volume  of 
the  ocean  current  when  we  note  how  trifling  peculiarities 
in  the  geography  of  the  shores — features  which  are  likely 
to  be  altered  by  the  endless  changes  which  occur  in  the 
form  of  a  continent — affect  the  run  of  these  currents. 
Thus  the  growth  of  coral  reefs  in  southern  Florida,  and, 
in  general,  the  formation  of  that  peninsula,  by  narrowing 
the  exit  of  the  great  current  from  the  Gulf  of  Mexico, 
has  probably  increased  its  velocity.  If  Florida  should  again 
sink  down,  that  current  would  go  forth  into  the  North 
Atlantic  with  the  speed  of  about  a  mile  an  hour,  and  would 
not  have  momentum  enough  to  carry  its  waters  over  half 
the  vast  region  which  they  now  traverse.  If  the  lands 
about  the  western  border  of  the  Caribbean  Sea,  particularly 
the  Isthmus  of  Darien,  should  be  depressed  to  a  consider- 


154  OUTLINES   OF  THE  EARTH'S  HISTORY. 

able  depth  below  the  ocean  level,  the  tropical  current 
Would  enter  the  Pacific  Ocean,  adding  to  the  temperature 
of  its  waters  all  the  precious  heat  which  now  vitalizes  the 
North  Atlantic  region.  Such  a  geographic  accident  would 
not  only  profoundly  alter  the  life  conditions  of  that  part 
of  the  world,  but  it  would  make  an  end  of  European  civi- 
lization. 

In  the  chapter  on  climatal  changes  further  atten- 
tion will  be  given  to  the  action  of  ocean  currents  from  the 
point  of  view  of  their  influence  on  the  heat  and  moisture 
of  different  parts  of  the  world.  We  now  have  to  consider 
the  last  important  influence  of  ocean  currents — that  which 
they  directly  exercise  on  the  development  of  organic  life. 
The  most  striking  effect  of  this  nature  which  the  sea 
streams  bring  about  is  caused  by  the  ceaseless  transporta- 
tion to  which  they  subject  the  eggs  and  seeds  of  animals 
and  plants,  as  well  as  the  bodies  of  the  mature  form  which 
are  moved  about  by  the  flowing  waters.  But  for  the  ex- 
istence of  these  north  and  south  flowing  currents,  due  to 
the  presence  of  the  continental  barriers,  the  living  tenants 
of  the  seas  would  be  borne  along  around  the  earth,  always 
in  the  same  latitude,  and  therefore  exposed  to  the  same 
conditions  of  temperature.  In  this  state  of  affairs  the  in- 
fluences which  now  make  for  change  in  organic  species 
would  be  far  less  than  they  are.  Journeying  in  the  great 
whirlpools  which  the  continental  barriers  make  out  of  the 
westward  setting  tropical  currents,  these  organic  species 
are  ever  being  exposed  to  alterations  in  their  temperature 
conditions  which  we  know  to  be  favourable  to  the  creation 
of  those  variations  on  which  the  advance  of  organic  life 
so  intimately  depends.  Thus  the  ocean  currents  not  only 
help  to  vary  the  earth  by  producing  changes  in  the  climate 
of  both  sea  and  land,  breaking  up  the  uniformity  which 
would  otherwise  characterize  regions  at  the  same  distance 
from  the  equator,  but  they  induce,  by  the  consequences  of 
the  migrations  which  they  enforce,  changes  in  the  organic 
tenants  of  the  sea. 


THE  ATMOSPHERE.  155 

Another  immediate  effect  of  ocean  streams  arises  where 
their  currents  of  warm  water  come  against  shores  or  shal- 
lows of  the  sea.  At  these  points,  if  the  water  have  a  trop- 
ical temperature,  we  invariably  find  a  vast  and  rapid  de- 
velopment of  marine  animals  and  plants,  of  which  the 
coral-making  polyps  are  the  most  important.  In  such 
positions  the  growth  of  forms  which  secrete  solid  skele- 
tons is  so  rapid  that  great  walls  of  their  remains  accumu- 
late next  the  shore,  the  mass  being  built  outwardly  by 
successive  growths  until  the  realm  of  the  land  may  be 
extended  for  scores  of  miles  into  the  deep.  In  other  cases 
vast  mounds  of  this  organic  debris  may  be  accumulated 
in  mid  ocean  until  its  surface  is  interspersed  with  myriads 
of  islands,  all  of  which  mark  the  work  due  to  the  combined 
action  of  currents  and  the  marine  life  which  they  nourish. 
Probably  more  than  four  fifths  of  all  the  islands  in  the 
tropical  belt  are  due  in  this  way  to  the  life-sustaining 
action  of  the  currents  which  the  trade  winds  create. 

There  are  many  secondary  influences  of  a  less  impor- 
tant nature  which  are  due  to  the  ocean  streams.  The 
reader  will  find  on  most  wall-maps  of  the  world  certain 
areas  in  the  central  part  of  the  oceans  w^hich  are  noted  as 
Sargassum  seas,  of  which  that  of  the  North  Atlantic,  west 
and  south  of  the  Azore  Islands,  is  one  of  the  most  conspicu- 
ous. In  these  tracts,  which  in  extent  may  almost  be  com- 
pared with  the  continents,  we  find  great  quantities  of  float- 
ing seaweed,  the  entangled  fronds  of  which  often  form  a 
mass  sufficiently  dense  to  slightly  restrain  the  speed  of 
ships.  When  the  men  on  the  caravels  of  Columbus  entered 
tliis  tangle,  they  were  alarmed  lest  they  should  be  unable 
to  escape  from  its  toils.  It  is  a  curious  fact  that  these 
weeds  of  the  sea  while  floating  do  not  reproduce  by  spores 
the  structures  which  answer  to  the  seeds  of  higher  plants, 
but  grow  only  by  budding.  It  seems  certain  that  they 
could  not  maintain  their  place  in  the  ocean  but  for  the 
action  of  the  currents  which  convey  the  bits  rent  off  from 
the  shores  where  the  plant  is  truly  at  home.     This  vast 


156  OUTLINES  OF  THE  EARTH'S  HISTORY. 

growth  of  plant  life  in  the  Sargassum  basins  doubtless  con- 
tributed considerable  and  important  deposits  of  sediment 
to  the  sea  floors  beneath  the  waters  which  it  inhabits. 
Certain  ancient  strata,  known  as  the  Devonian  black  shale, 
occupying  the  Ohio  valley  and  the  neighbouring  parts  of 
North  America  to  the  east  and  north  of  that  basin,  appear 
to  be  accumulations  which  w^ere  made  beneath  an  ancient 
Sargassum  sea. 

The  ocean  currents  have  greatly  favoured  and  in  many 
instances  determined  the  migrations  not  only  of  marine 
forms,  but  of  land  creatures  as  well.  Floating  timber  may 
bear  the  eggs  and  seeds  of  many  forms  of  life  to  great  dis- 
tances until  the  rafts  are  cast  ashore  in  a  realm  where,  if 
the  conditions  favour,  the  creatures  may  find  a  new  seat  for 
their  life.  Seeds  of  plants  incased  in  their  often  dense 
envelopes  may,  because  they  float,  be  independently  car- 
ried great  distances.  So  it  comes  about  that  no  sooner 
does  a  coral  or  other  island  rise  fbove  the  waters  of  the 
sea  than  it  becomes  occupied  by  a  varied  array  of  plants. 
The  migrations  of  people,  even  down  to  the  time  of  the 
voyages  which  discovered  America,  have  in  large  measure 
been  controlled  by  the  run  of  the  ocean  streams.  The 
tropical  set  of  the  waters  to  the  westward  helped  Colum- 
bus on  his  way,  and  enabled  him  to  make  a  journey  which 
but  for  their  assistance  could  hardly  have  been  accom- 
plished. This  same  current  in  the  northern  part  of  the 
Gulf  Stream  opposed  the  passage  of  ships  from  northern 
Europe  to  the  westward,  and  to  this  day  affects  the  speed 
with  which  their  voyages  are  made. 

The  Circuit  of  the  Eain. 

We  have  now  to  consider  those  movements  of  the  water 
which  depend  upon  the  fact  that  at  ordinary  temperatures 
the  sea  yields  to  the  air  a  continued  and  large  supply  of 
vapour,  a  contribution  which  is  made  in  lessened  propor- 
tion by  water  in  all  stages  of  coldness,  and  even  by  ice 


THE  ATMOSPHERE.  157 

when  it  is  exposed  to  dry  air.  This  evaporation  of  the  cea 
water  is  proportional  to  the  temperature  and  to  the  dry- 
ness of  the  air  where  it  rests  upon  the  ocean.  It  prob- 
ably amounts  on  the  average  to  somewhere  about  three 
feet  per  annum;  in  regions  favourably  situated  for  the 
process,  as  on  the  west  coast  of  northern  Africa,  it  may 
be  three  or  four  times  as  much,  while  in  the  cold  and 
humid  air  about  the  poles  it  may  be  as  little  as  one  foot. 
AVhen  contributed  to  the  air,  the  water  enters  on  the  state 
of  vapour,  in  which  state  it  tends  to  diffuse  itself  freely 
through  the  atmosphere  by  virtue  of  the  motion  w^hich  is 
developed  in  particles  when  in  the  vaporous  or  gaseous 
state. 

The  greater  part  of  the  water  evaporated  from  the 
seas  probably  finds  its  way  as  rain  at  once  back  into  the 
deep,  yet  a  considerable  portion  is  borne  away  horizontally 
until  it  encounters  the  land.  The  precipitation  of  the 
water  from  the  air  is  primarily  due  to  the  cooling  to  which 
it  is  subjected  as  it  rises  in  the  atmosphere.  Over  the  sea 
the  ascent  is  accomplished  by  the  simple  diffusion  of  the 
vapour  or  by  the  uprise  through  the  aerial  shaft,  such  as 
that  near  the  equator  or  over  the  centres  of  the  whirling 
storms.  It  is  when  the  air  strikes  the  slopes  of  the  land 
that  we  find  it  brought  into  a  condition  which  most  de- 
cidedly tends  to  precipitate  its  moisture.  Lifted  upward, 
the  air  as  it  ascends  the  slopes  is  brought  into  cooler  and 
more  rarefied  conditions.  Losing  temperature  and  expand- 
ing, it  parts  with  its  water  for  the  same  reason  that  it  does 
in  the  ascending  current  in  the  equatorial  belt  or  in  the 
chimneys  of  the  whirl  storms.  A  general  consequence  of 
this  is  that  wherever  moisture-laden  winds  from  the  sea 
impinge  upon  a  continent  they  lay  down  a  considerable 
part  of  the  water  which  they  contain. 

If  all  the  lands  were  of  the  same  height,  the  rain  would 
generally  come  in  largest  proportion  upon  their  coastal 
belt,  or  those  portions  of  the  shore-line  districts  over  which 
the  sea  winds  swept.     But  as  these  winds  vary  in  the 


158  OUTLINES  OF  THE  EARTH'S  HISTORY. 

amount  of  the  watery  vapour  which  they  contain,  and  as 
the  surface  of  the  land  is  very  irregular,  the  rainfall  is 
the  most  variable  feature  in  the  climatal  conditions  of  our 
sphere.  Near  the  coasts  it  ranges  from  two  or  three  inches 
in  arid  regions — such  as  the  western  part  of  the  Sahara 
and  portions  of  the  coast  regions  of  Chili  and  Peru — to 
eight  hundred  inches  about  the  head  waters  of  the  Brahma- 
pootra River  in  northern  India,  where  the  high  moun- 
tains are  swept  over  by  the  moisture-laden  airs  from  the 
neighbouring  sea.  Here  and  there  detached  mountainous 
masses  produce  a  singular  local  increase  in  the  amount  of 
the  rainfall.  Thus  in  the  lake  district  in  northwestern 
England  the  rainfall  on  the  seaward  side  of  mountains, 
not  over  four  thousand  feet  high,  is  very  much  greater 
than  it  is  on  the  other  slope,  less  than  a  score  of  miles 
away.  These  local  variations  are  common  all  over  the 
world,  though  they  are  but  little  observed. 

In  general,  the  central  parts  of  continents  are  likely 
to  receive  much  less  rainfall  than  their  peripheral  por- 
tions. Thus  the  central  districts  of  North  America,  Asia, 
and  Australia — three  out  of  the  five  continental  masses — 
have  what  we  may  call  interior  deserts.  Africa  has  one 
such,  though  it  is  north  of  the  centre,,  and  extends  to  the 
shores  of  the  Mediterranean  and  the  Atlantic.  The  only 
continent  without  this  central  nearly  rainless  field  is  South 
America,  where  the  sole  characteristic  arid  district  is  situ- 
ated on  the  western  slope  of  the  Cordilleran  range.  In 
this  case  the  peculiarity  is  due  to  the  fact  that  the  strong 
westerly  setting  winds  which  sweep  over  the  country  en- 
counter no  high  mountains  until  they  strike  the  Andean 
chain.  They  journey  up  a  long  and  rather  gradual  slope, 
where  the  precipitation  is  gradually  induced,  the  process 
being  completed  when  they  strike  the  mountain  wall. 
Passing  over  its  summit,  they  appear  as  dry  winds  on  the 
Pacific  coast. 

Even  while  the  winds  frequently  blow  in  from  the  sea, 
as  along  the  western  coast  of  the  Americas,  they  may  come 


THE  ATMOSPHERE.  159 

over  water  which  is  prevailingly  colder  than  the  land. 
This  is  characteristically  the  case  on  the  western  faces  of 
the  American  continent,  where  the  sea  is  cooled  by  the 
currents  setting  toward  the  equator  from  high  latitudes. 
Such  cool  sea  air  encountering  the  warm  land  has  its  tem- 
perature raised,  and  therefore  does  not  tend  to  lay  down 
its  burden  of  moisture,  but  seeks  to  take  up  more.  On  this 
account  the  rainfall  in  countries  placed  under  such  con- 
ditions is  commonly  small. 

By  no  means  all  the  moisture  which  comes  upon  the 
earth  from  the  atmosphere  descends  in  the  form  of  rain 
or  snow.  A  variable,  large,  though  yet  undetermined 
amount  falls  in  the  form  of  dew.  Dew  is  a  precipitation 
of  moisture  which  has  not  entered  the  peculiar  state  which 
we  term  fog  or  cloud,  but  has  remained  invisible  in  the 
air.  It  is  brought  to  the  earth  through  the  radiation  of 
heat  which  continually  takes  place,  but  which  is  most 
effective  during  the  darkened  half  of  the  day,  when  the 
action  is  not  counterbalanced  by  the  sun's  rays.  ^\Tiile  the 
sun  is  high  and  the  air  is  warm  there  is  a  constant  ab- 
sorption of  moisture  in  large  part  from  the  ground  or 
from  the  neighbouring  water  areas,  probably  in  some  part 
from  those  suspended  stores  of  water,  the  clouds,  if  such 
there  be  in  the  neighbourhood.  We  can  readily  notice 
how  clouds  drifting  in  from  the  sea  often  melt  into  the 
dry  air  which  they  encounter.  Late  in  the  afternoon, 
even  before  the  sun  has  sunk,  the  radiation  of  heat  from 
the  earth,  which  has  been  going  on  all  the  while,  but  has 
been  less  considerable  than  the  incurrent  of  temperature, 
in  a  way  overtakes  that  influx.  The  air  next  the  surface 
becomes  cooled  from  its  contact  with  the  refrigerating 
earth,  and  parts  with  its  moisture,  forming  a  coating  of 
water  over  everything  it  touches.  At  the  same  time  the 
moisture  escaping  from  the  warmed  under  earth  likewise 
drops  back  upon  its  cooled  surface  almost  as  soon  as  it  has 
escaped.  The  thin  sheet  of  water  precipitated  by  this 
method  is  quickly  returned  to  the  air  when  it  becomes 


160  OUTLINES  OF  THE  EARTH'S  HISTORY. 

warmed  by  the  morning  sunshine,  but  during  the  night 
quantities  of  it  are  absorbed  by  the  plants;  very  often, 
indeed,  with  the  lowlier  vegetation  it  trickles  down  the 
leaves  and  enters  the  earth  about  the  base  of  the  stem, 
so  that  the  roots  may  appropriate  it.  Our  maize,  or  Indian 
corn,  affords  an  excellent  example  of  a  plant  which,  hav- 
ing developed  in  a  land  of  droughts,  is  well  contrived, 
through  its  capacities  for  gathering  dew,  to  protect  itself 
against  arid  conditions.  In  an  ordinary  dew-making  night 
the  leaves  of  a  single  stem  may  gather  as  much  as  half  a 
pint  of  water,  which  flows  down  their  surfaces  to  the  roots. 
So  efficient  is  this  dew  supply,  this  nocturnal  cloudless 
rain,  that  on  the  western  coast  of  South  America  and  else- 
where, where  the  ordinary  supply  of  moisture  is  almost 
wanting,  many  important  plants  are  able  to  obtain  from 
it  much  of  the  water  which  they  need.  The  effect  is  par- 
ticularly striking  along  seashores,  where  the  air,  although 
it  may  not  have  the  humidity  necessary  for  the  formation 
of  rain,  still  contains  enough  to  form  dew. 

It  is  interesting  to  note  that  the  quantity  of  dew  which 
falls  upon  an  area  is  generally  proportioned  to  the  amount 
of  living  vegetation  which  it  bears.  The  surfaces  of  leaves 
are  very  efficient  agents  of  radiation,  and  the  tangle  which 
they  make  offers  an  amount  of  heat-radiating  area  many 
times  as  great  as  that  afforded  by  a  surface  of  bared  earth. 
Moreover,  the  ground  itself  can  not  well  cool  down  to  the 
point  where  it  will  wring  the  moisture  out  of  the  air,  while 
the  thin  membranes  of  the  plants  readily  become  so  cooled. 
Thus  vegetation  by  its  own  structure  provides  itself  with 
means  whereby  it  may  be  in  a  measure  independent  of  the 
accidental  rainfall.  We  should  also  note  the  fact  that 
the  dewfall  is  a  concomitant  of  cloudless  skies.  The  quan- 
tity which  is  precipitated  in  a  cloudy  night  is  very  small, 
and  this  for  the  reason  that  when  the  heavens  are  covered 
the  heat  from  the  earth  can  not  readily  fly  off  into  space. 
Under  these  conditions  the  temperature  of  the  air  rarely 
descends  low  enough  to  favour  the  precipitation  of  dew. 


THE  ATMOSPHERE.  161 

Having  noted  the  process  by  which  in  the  rain  circuit 
the  water  leaves  the  sea  and  the  conditions  of  distribution 
when  it  returns  to  the  earth,  we  may  now  trace  in  more 
detail  the  steps  in  this  great  round.  First,  we  should  take 
note  of  the  fact  that  the  water  after  it  enters  the  air  may 
come  back  to  the  surface  of  the  earth  in  either  of  two 
ways — directly  in  the  manner  of  dewfall,  or  in  a  longer 
circuit  which  leads  it  through  the  state  of  clouds.  As 
yet  we  are  not  very  well  informed  as  to  the  law  of  the 
cloud-making,  but  certain  features  in  this  picturesque  and 
most  important  process  have  been  tolerably  well  ascer- 
tained. 

Eising  upward  from  the  sea,  the  vapour  of  w^ater  com- 
monly remains  transparent  and  invisible  until  it  attains 
a  considerable  height  above  the  surface,  where  the  cooling 
tends  to  make  it  assume  again  the  visible  state  of  cloud 
particles.  The  formation  of  these  cloud  particles  is  now 
believed  to  depend  on  the  fact  that  the  air  is  full  of  small 
dust  motes,  exceedingly  small  bits  of  matter  derived  from 
the  many  actions  which  tend  to  bring  comminuted  solid 
matter  into  the  air,  as,  for  instance,  the  combustion  of 
meteoric  stones,  which  are  greatly  heated  by  friction  in 
their  swift  course  through  the  air,  the  ejections  of  vol- 
canoes, the  smoke  of  forest  and  other  fires,  etc.  These  tiny 
bits,  floating  in  the  air,  because  of  their  solid  nature  radi- 
ate their  heat,  cool  the  air  which  lies  against  them,  and 
thereby  precipitate  the  water  in  the  manner  of  dew,  exactly 
as  do  the  leaves  and  other  structures  on  the  surface  of  the 
earth.  In  fact,  dew  formation  is  essentially  like  cloud 
formation,  except  that  in  the  one  case  the  water  is  gathered 
on  fixed  bodies,  and  in  the  other  on  floating  objects.  Each 
little  dust  raft  with  its  cargo  of  condensed  water  tends,  of 
course,  to  fall  downward  toward  the  earth's  surface,  and, 
except  for  the  winds  which  may  blow  upward,  does  so 
fall,  though  with  exceeding  slowness.  Its  rate  of  descent 
may  be  only  a  few  feet  a  day.  It  was  falling  before  it  took 
on  the  load  of  water;  it  will  fall  a  little  more  rapidly 


10,2  OUTLINES  OF   THE  EARTH'S  HISTORY. 

with  the  added  burden,  but  even  in  a  still  air  it  might 
be  months  or  years  before  it  would  come  to  the  ground. 
The  reason  for  this  slow  descent  may  not  at  first  sight  be 
plain,  though  a  little  consideration  will  make  it  so. 

If  we  take  a  shot  of  small  size  and  a  feather  of  the 
same  weight,  we  readily  note  that  their  rate  of  falling 
through  the  air  may  vary  in  the  proportion  of  ten  to  one 
or  more.  It  is  easy  to  conceive  that  this  difference  is  due 
to  the  very  much  less  friction  which  the  smaller  body  en- 
counters in  its  motion  by  the  particles  of  air.  With  this 
point  in  mind,  the  student  should  observe  that  the  sur- 
face presented  by  solid  bodies  in  relation  to  their  solid 
contents  is  the  greater  the  smaller  the  diameter.  A  rough, 
though  not  very  satisfactory,  instance  of  this  principle  may 
be  had  by  comparing  the  surface  and  interior  contents 
of  two  boxes,  one  ten  feet  square  and  the  other  one  foot 
square.  The  larger  has  six  hundred  feet  of  surface  to  one 
thousand  cubic  feet  of  interior,  or  about  half  a  square  foot 
of  outer  surface  to  the  cubic  foot  of  contents;  while  the 
smaller  box  has  six  feet  of  surface  for  the  single  cubic 
foot  of  interior,  or  about  ten  times  the  proportion  of  ex- 
terior to  contents.  The  result  is  that  the  smaller  particles 
encounter  more  friction  in  moving  toward  the  earth,  until, 
in  the  case  of  finely  divided  matter,  such  as  the  particles 
of  carbon  in  the  smoke  from  an  ordinary  fire,  the  rate  of 
down-falling  may  be  so  small  as  to  have  little  effect  in  the 
turbulent  conditions  of  atmospheric  motion. 

The  little  drops  of  water  which  gather  round  dust  motes, 
falling  but  slowly  toward  the  earth,  are  free  to  obey  the 
attractions  which  they  exercise  upon  each  other — impulses 
which  are  partly  gravitative  and  partly  electrical.  We 
have  no  precise  knowledge  concerning  these  movements, 
further  than  that  they  serve  to  aggregate  the  myriad  little 
floats  into  cloud  forms,  in  which  the  rafts  are  brought  near 
together,  but  do  not  actually  touch  each  other.  They  are 
possibly  kept  apart  by  electrical  repulsion.  In  this  state 
of  association  without  union  the  divided  water  may  under- 


THE  ATMOSPHERE.  163 

go  the  curiously  modified  aggregations  which  give  us  the 
varied  forms  of  clouds.  As  yet  we  know  little  as  to  the 
cause  of  cloud  shapes.  We  remark  the  fact  that  in  the 
higher  of  these  agglomerations  of  condensed  vapour,  the 
clouds  which  float  at  an  elevation  of  from  twenty  to  thirty 
thousand  feet  or  more,  the  masses  are  generally  thin,  and 
arranged  more  or  less  in  a  leaflike  form,  though  even  here 
a  tendency  to  produce  spherical  clouds  is  apparent.  In 
this  high  realm  floating  water  is  probably  in  the  frozen 
state,  answering  to  the  form  of  dew,  which  we  call  hoar 
frost.  The  lower  clouds,  gathering  in  the  still  air,  show 
very  plainly  the  tendency  to  agglomerate  into  spheres, 
which  appears  to  be  characteristic  of  all  vaporous  material 
which  is  free  to  move  by  its  own  impulses.  It  is  probable 
that  the  spherical  shape  of  clouds  is  more  or  less  due  to 
the  same  conditions  as  gathered  the  stellar  matter  from  the 
ancient  nebular  chaos  into  the  celestial  spheres.  Upon 
these  spherical  aggregations  of  the  clouds  the  winds  act 
in  extremely  varied  ways.  The  cloud  may  be  rubbed  be- 
tween opposite  currents,  and  so  flattened  out  into  a  long 
streamer;  it  may  take  the  same  form  by  being  carried  off 
by  a  current  in  the  manner  of  smoke  from  a  fire;  the 
spheres  may  be  kept  together,  so  as  to  form  the  patchwork 
which  we  call  "  mackerel "  sky;  or  they  may  be  actually 
confounded  with  each  other  in  a  vast  common  cloud-heap. 
In  general,  where  the  process  of  aggregation  of  two  cloud 
bodies  occurs,  changes  of  temperature  are  induced  in  the 
masses. which  are  mixed  together.  If  the  temperature  re- 
sulting from  this  association  of  cloud  masses  is  an  average 
increase,  the  cloud  may  become  lighter,  and  in  the  manner 
of  a  balloon  move  upward.  Each  of  the  motes  in  the  cloud 
with  its  charge  of  vapour  may  be  compared  with  the  bal- 
last of  the  balloon;  if  they  are  warmed,  they  send  forth 
a  part  of  their  load  of  condensed  water  again  to  the  state 
of  invisible  vapour.  Eising  to  a  point  where  it  cools,  the 
vapour  gathers  back  on  the  rafts  and  tends  again  to 
^weight  the  cloud  downward.     The  ballast  of  an  ordinary 


164:  OUTLINES  OF  THE  EARTH'S  HISTORY. 

balloon  has  to  be  thrown  away  from  its  car;  but  if  some 
arrangement  for  condensing  the  moisture  from  the  air 
could  be  contrived,  a  balloon  might  be  brought  into  the 
adjustable  state  of  a  cloud,  goiiig  "np  or  down  according 
as  it  was  heated  or  cooled. 

When  the  formation  of  the  drop  of  water  or  snowflake 
begins,  the  mass  is  very  small.  If  in  descending  it  encoun- 
ters great  thickness  of  cloud,  the  bit  may  grow  by  further 
condensation  until  it  becomes  relatively  large.  Gener- 
ally in  this  way  we  may  account  for  the  diversities  in  the 
size  of  raindrops  or  snowflakes.  It  often  happens  that 
the  particles  after  taking  on  the  form  of  snowflakes  en- 
counter in  their  descent  air  so  warm  that  they  melt  into 
raindrops,  or,  if  only  partly  melted,  reach  the  surface  as 
sleet.  Or,  starting  as  raindrops,  they  may  freeze,  and  in 
this  simple  state  may  reach  the  earth,  or  after  freezing 
they  may  gather  other  frozen  water  about  them,  so  that 
the  hailstone  has  a  complicated  structure  which,  from  the 
point  of  view  of  classification,  is  between  a  raindrop  and 
a  snowflake. 

In  the  process  of  condensation — indeed,  in  the  steps 
which  precede  the  formation  of  rain  and  snow — there  is 
often  more  or  less  trace  of  electrical  action;  in  fact,  a  part 
of  the  energy  which  was  involved  in  the  vapourization  of 
water,  on  its  condensation,  even  on  the  dust  motes  appears 
to  be  converted  into  electrical  action,  which  probably  op- 
erates in  part  to  keep  the  little  aggregates  of  water  asun- 
der. When  they  coalesce  in  drops  or  flakes,  this  electric- 
ity often  assumes  the  form  of  lightning,  which  represents 
the  swift  passage  of  the  electric  store  from  a  region  where 
it  is  most  abundant  to  one  where  it  is  less  so.  The  varia- 
tions in  this  process  of  conveying  the  electricity  are  prob- 
ably great.  In  general,  it  probably  passes,  much  as  an 
electric  current  is  conveyed,  through  a  wire  from  the  bat- 
tery which  produces  the  force.  In  other  cases,  where  the 
tension  is  high,  or,  in  other  words,  where  the  discharge 
has  to  be  hastened,  we  have  the  phenomena  of  lightning 


THE  ATMOSPHERE.  165 

in  which  the  current  burns  its  way  along  its  path,  as  it 
may  traverse  a  slender  wire,  vapourizing  it  as  it  goes.  In 
general,  the  lightning  flash  expends  its  force  on  the  air 
conductors,  or  lines  of  the  moist  atmosphere  along  which 
it  breaks  its  path,  its  energy  returning  into  the  vapour 
which  it  forms  or  the  heat  which  it  produces  in  the  other 
parts  of  the  air.  In  some  cases,  probably  not  one  in  the 
thousand  of  the  flashes,  the  charge  is  so  heavy  that  it  is 
not  used  up  in  its  descent  toward  the  earth,  and  so  electri- 
fies, or,  as  we  say,  strikes,  some  object  attached  to  the 
earth,  through  which  it  passes  to  the  underlying  moisture, 
where  it  finds  a  convenient  place  to  take  on  a  quiet  form. 
Almost  all  these  hurried  movements  of  electrical  energy 
which  intensely  heat  and  light  the  air  which  they  traverse 
fly  from  one  part  of  a  cloud  to  another,  or  cross  from  cloud 
sphere  to  cloud  sphere;  of  those  which  start  toward  the 
earth,  many  are  exhausted  before  they  reach  its  surface, 
and  even  those  that  strike  convey  but  a  portion  of  their 
original  impulse  to  the  ground. 

The  wearing-out  effect  of  lightning  in  its  journey 
along  the  air  conductors  in  its  flaming  passages  is  well 
illustrated  by  what  happens  when  the  charge  strikes  a 
wire  which  is  not  large  enough  freely  to  convey  it.  The 
wire  is  heated,  generally  made  white  hot,  often  melted, 
and  perhaps  scattered  in  the  form  of  vapour.  In  doing 
this  work  the  electricity  may,  and  often  is,  utterly  dissi- 
pated— that  is,  changed  into  heat.  It  has  been  proposed 
to  take  advantage  of  this  principle  in  protecting  buildings 
from  lightning  by  placing  in  them  many  thin  wires,  along 
which  the  current  will  try  to  make  its  way,  being  ex- 
hausted in  melting  or  vaporizing  the  metal  through  which 
it  passes. 

There  are  certain  other  forms  of  lightning,  or  at  least 
of  electrical  discharges,  which  produce  light  and  which  may 
best  be  described  in  this  connection.  ^  It  occasionally  hap- 
pens that  the  earth  becomes  so  charged  that  the  current 
proceeds  from  its  surface  to  the  clouds.    More  rarely,  and 


166  OUTLINES  OP  THE  EARTH'S  HISTORY. 

Tinder  conditions  which  we  do  not  understand,  the  elec- 
tric energy  is  gathered  into  a  ball-like  form,  which  may 
move  slowly  along  the  surface  until  it  suddenly  explodes. 
It  is  a  common  feature  of  all  these  forms  of  liglitning 
which  we  have  noted  that  they  ordinarily  make  in  their 
movement  considerable  noise.  This  is  due  to  the  sudden 
displacement  of  the  air  which  they  traverse — displace- 
ment due  to  the  action  of  heat  in  separating  the  particles. 
It  is  in  all  essential  regards  similar  to  the  sounds  made 
by  projectiles,  such  as  meteors  or  swift  cannon  shots,  as 
they  fly  through  the  air.  It  is  even  more  comparable  to 
the  sound  produced  by  exploding  gunpowder.  Tlie  first 
sound  effect  from  the  lightning  stroke  is  a  single  rending 
note,  which  endures  no  longer — indeed,  not  as  long — as 
the  explosion  of  a  cannon.  Heard  near  by,  this  note  is 
very  sharp,  reminding  one  of  the  sound  made  by  the 
breaking  of  glass.  The  rolling,  continuous  sound  which 
we  commonly  hear  in  thunder  is,  as  in  the  case  of  the 
noise  produced  by  cannon,  due  to  echo  from  the  clouds 
and  the  earth.  Thunder  is  ordinarily  much  more  pro- 
longed and  impressive  in  a  mountainous  country  than  in 
a  region  of  plains,  because  the  steeps  about  the  hearer 
reverberate  the  original  single  crash. 

The  distribution  of  thunderstorms  is  as  yet  not  well 
understood,  but  it  appears  in  many  cases  that  they  are 
attendants  on  the  advancing  face  of  cyclones  and  hurri- 
canes, the  area  in  front  of  these  great  whirlstorms  being 
subjected  to  the  condensation  and  irregular  air  movements 
which  lead  to  the  development  of  much  electrical  energy. 
There  are,  however,  certain  parts  of  the  earth  which  are 
particularly  subjected  to  lightning  flashes.  They  are 
common  in  the  region  near  the  equator,  where  the  ascend- 
ing currents  bring  about  heavy  rains,  which  mean  a  rapid 
condensation  and  consequent  liberation  of  electrical  en- 
ergy. They  diminish  in  frequency  toward  the  arctic 
regions.  An  observer  at  the  pole  would  probably  fail  ever 
to  perceive  strong  flashes.    For  the  same  reason  thunder- 


THE  ATMOSPHERE.  167 

storms  are  more  frequent  in  summer,  the  time  when  the 
difference  in  temperature  between  the  surface  and  the 
upper  air  is  greatest,  when,  therefore,  the  uprushes  of  air 
are  Hkely  to  be  most  violent.  They  appear  to  be  more 
common  in  the  night  than  in  the  daytime,  for  the  reason 
that  condensation  is  favoured  by  the  cooling  which  occurs 
in  the  dark  half  of  the  day.  It  is  rare,  indeed,  that  a 
thunderstorm  occurs  near  midday,  a  period  when  the  air 
is  in  most  cases  taking  up  moisture  on  account  of  the 
swiftly  increasing  heat. 

There  are  other  forms  of  electrical  discharges  not  dis- 
tinctly connected  with  the  then  existing  condensation  of 
moisture.  What  the  sailors  call  St.  Elmo's  fire — a  brush 
of  electric  light  from  the  mast  tops  and  other  projections 
of  the  ship — indicates  the  passage  of  electrical  energy  be- 
tween the  vessel  and  the  atmosphere.  Similar  lights  are 
said  sometimes  to  be  seen  rising  from  the  surface  of  the 
water.  Such  phenomena  are  at  present  not  satisfactorily 
explained.  Perhaps  in  the  same  group  of  actions  comes  the 
so-called  "  Jack-o'-lantern  "  or  "  Will-o'-the-wisp  "  fires 
flashing  from  the  earth  in  marshy  places,  which  are  often 
described  by  the  common  people,  but  have  never  been 
observed  by  a  naturalist.  If  this  class  of  illuminations 
really  exists,  we  have  to  afford  them  some  other  explana- 
tion than  that  they  are  emanations  of  self-inflamed  phos- 
phuretted  hydrogen,  a  method  of  accounting  for  them 
which  illogically  finds  a  place  in  many  treatises  on  atmos- 
pheric phenomena.  A  gas  of  any  kind  would  disperse  itself 
in  the  air;  it  could  not  dance  about  as  these  lights  are  said 
to  do,  and  there  is  no  chemical  means  known  whereby  it 
could  be  produced  in  sufficient  purity  and  quantity  from 
the  earth  to  produce  the  effects  which  are  described.* 

*  The  present  writer  has  made  an  extended  and  careful  study  of 

marsh  and  swamp  phenomena,  and  is  very  familiar  with  the  aspect  of 

these  fields  in  the  nighttime.     He  has  never  been  able  to  see  any  sign 

of  the  Jack-o'-lantern  h'ght.     Looking  fixedly  into  any  darkness,  such 

12 


168  OUTLINES  OF  THE  EARTH'S  HISTORY. 

In  the  upper  air,  or  perhaps  even  beyond  the  limits  of 
the  field  which  deserves  the  name,  in  the  regions  extend- 
ing from  the  poles  to  near  the  tropics,  there  occur  elec- 
tric glo wings  commonly  known  as  the  aurora  borealis. 
This  phenomenon  occurs  in  both  hemispheres.  These  illu- 
minations, though  in  some  way  akin  to  those  of  lightning, 
and  though  doubtless  due  to  some  form  of  electrical  action, 
are  peculiar  in  that  they  are  often  attended  by  glows  as  if 
from  clouds,  and  by  pulsations  which  indicate  movements 
not  at  electric  speed.  As  yet  but  little  is  known  as  to  the 
precise  nature  of  these  curious  storms.  It  has  been  claimed, 
however,  that  they  are  related  to  the  sun  spots;  those  peri- 
ods when  the  solar  spots  are  plenty,  at  intervals  of  about 
eleven  years,  are  the  times  of  auroral  discharges.  Still 
further,  it  seems  probable  that  the  magnetic  currents  of  the 
earth,  that  circling  energy  which  encompasses  the  sphere, 
moving  round  in  a  general  way  parallel  to  the  equator,  are 
intensified  during  these  illuminations  of  the  circumpolar 
skies. 

Geological  Work  of  Water. 

We  turn  now  to  the  geological  work  which  is  performed 
by  falling  water.  Where  the  rain  or  snow  returns  from 
the  clouds  to  the  sea,  the  energy  of  position  given  to  the 
water  by  its  elevation  above  the  earth  through  the  heat 
which  it  acquired  from  the  sun  is  returned  to  the  air 
through  which  it  falls  or  to  the  ocean  surface  on  which 
it  strikes.    In  this  case  the  circuit  of  the  rain  is  short  and 

as  is  afforded  by  the  depths  of  a  wood,  the  eye  is  apt  to  imagine  the 
appearance  of  faint  lights.  Those  who  have  had  to  do  with  outpost 
duty  in  an  army  know  how  the  anxious  sentry,  particularly  if  he  is 
new  to  the  soldier's  trade,  will  often  imagine  that  he  sees  lights  be- 
fore him.  Sometimes  the  pickets  will  be  so  convinced  of  the  fact 
that  they  see  lights  that  they  will  fire  upon  the  fiction  of  the  imagi- 
nations. These  facts  make  it  seem  probable  that  the  Jack-o'-lantern 
and  his  companion,  the  Will-o'-the-wisp,  are  stories  of  the  over- 
credulous. 


THE  ATMOSPHERE. 


169 


without  geological  consequence  which  it  is  worth  while 
to  consider,  except  to  note  that  the  heat  thus  returned  is 
likely  to  be  delivered  in  another  realm  than  that  in  which 
the  falling  water  acquired  the  store,  thus  in  a  small  way 
modifying  the  climate.  When,  however,  the  precipitation 
occurs  on  the  surface  of  the  land,  the  drops  of  frozen  or 
fluid  water  apply  a  part  of  their  energy  in  important  geo- 
logical work,  the  like  of  which  is  not  done  where  they 
return  at  once  to  the  sea. 

We  shall  first  consider  what  takes  place  when  the 
water  in  the  form  of  drops  of  rain  comes  to  the  surface  of 
the  land.  Descending  as  they  do  with  a  considerable 
speed,  these  raindrops  apply  a  certain  amount  of  energy 


Fia.  10. — Showing  the  diverse  action  of  rain  on  wooded  and  cleared 
fields,    a,  wooded  area ;  b,  tilled  ground. 


to  the  surface  on  which  they  fall.  Although  the  beat  of 
a  raindrop  is  proverbially  light,  the  stroke  is  not  ineffect- 
ive. Observing  what  happens  where  the  action  takes 
place  on  the  surface  of  bare  rock,  we  may  notice  that  the 
grains  of  sand  or  small  pebbles  which  generally  abound 
on  such  surfaces,  if  they  be  not  too  steeply  inclined,  dance 
about  under  the  blows  which  they  receive.  If  we  could 
cover  hard  plate  glass,  a  much  firmer  material  than  ordi- 
nary stone,  with  such  bits,  we  should  soon  find  that  its 


170  OUTLINES  OP  THE  EARTH'S  HISTORY. 

surface  would  become  scratched  all  over  by  the  friction. 
Moreover,  the  raindro^^s  perceptibly  urge  the  small  de- 
tached bits  of  stone  down  the  slopes  toward  the  streams. 

If  all  the  earth's  surface  were  bare  rocks,  the  blow  of 
the  raindrops  would  deserve  to  be  reckoned  among  the 
important  influences  which  lead  to  the  wearing  of  land. 
As  it  is,  when  a  country  is  in  a  state  of  Nature,  only  a 
small  part  of  its  surface  is  exposed  to  this  kind  of  wear- 
ing. Where  there  is  rain  enough  to  effect  any  damage, 
there  is  sure  to  be  sufficient  vegetation  to  interpose  a  living 
and  self -renewed  covering  between  the  rocks  and  the  rain. 
Even  the  lichens  which  coat  what  at  first  sight  often  seems 
to  be  bare  rock  afford  an  ample  covering  for  this  purpose. 
It  is  only  where  man  bares  the  field  by  stripping  away 
and  overturning  this  protecting  vegetation  that  the  rain- 
drops cut  away  the  earth.  The  effect  of  their  action  can 
often  be  noted  by  observing  how  on  ploughed  ground  a 
flat  stone  or  a  potsherd  comes  after  a  rain  to  cap  a  little 
column.  The  geologist  sometimes  finds  in  soft  sandstones 
that  the  same  action  is  repeated  in  a  larger  way  where 
a  thin  fragment  of  hard  rock  has  protected  a  column  many 
feet  in  height  against  the  rain  work  which  has  shorn  down 
the  surrounding  rock. 

When  water  strikes  the  moistened  surface  it  at  once 
loses  the  droplike  form  which  all  fluids  assume  when  they 
fall  through  the  air.* 

When  the  raindrops  coalesce  on  the  surface  of  the 
earth,  the  role  of  what  we  may  call  land  water  begins. 

*  This  principle  of  the  spheroidal  form  in  falling  fluids  is  used 
(n  making  ordinary  bird  shot.  The  melted  lead  drops  through 
sievelike  openings,  the  resulting  spheres  of  the  metal  being  allowed 
to  fall  into  water  which  chills  them.  Iron  shot,  used  in  cutting 
stone,  where  they  are  placed  between  the  saw  and  the  surface  of 
the  rock,  are  also  made  in  the  same  manner.  The  descending  fluid 
divides  into  drops  because  it  is  drawn  out  by  the  ever-increasing 
speed  of  the  falling  particles,  which  soon  make  the  stream  so  thin 
that  it  ean  not  hold  together. 


THE  ATMOSPHERE.  171 

Thenceforward  until  the  fluid  arrives  at  the  surface  of  the 
sea  it  is  continually  at  work  in  effecting  a  great  range  of 
geological  changes,  only  a  few  of  which  can  well  be  traced 
by  the  general  student.  The  work  of  land  water  is  due 
to  three  classes  of  properties — to  the  energy  with  which 
it  is  endowed  by  virtue  of  its  height  above  the  sea,  a  power 
due  to  the  heat  of  the  sun;  to  the  capacity  it  has  for  taking 
substances  into  solution;  and  to  its  property  of  giving  some 
part  of  its  own  substance  to  other  materials  with  which  it 
comes  in  contact.  The  first  of  these  groups  of  properties 
may  be  called  dynamical;  the  others,  chemical. 

The  dynamic  value  of  water  when  it  falls  upon  the 
land  is  the  amount  of  energy  it  can  apply  in  going  down 
the  slope  which  separates  it  from  the  sea.  A  ton  of  the 
fluid,  such  as  may  gather  in  an  ordinary  rain  on  a  thou- 
sand square  feet  of  ground  in  the  highlands  of  a  country — 
say  at  an  elevation  of  a  thousand  feet  above  the  sea — ex- 
pends before  it  comes  to  rest  in  the  great  reservoir  as 
much  energy  as  would  be  required  to  lift  that  weight  from 
the  ocean's  surface  to  the  same  height.  The  ways  in  which 
this  energy  may  be  expended  we  shall  now  proceed  in  a 
general  way  to  trace. 

As  soon  as  the  water  has  been  gathered,  from  its  drop 
to  its  sheet  state — a  process  which  takes  place  as  soon  as 
it  falls — the  fluid  begins  its  downward  journey.  On  this 
way  it  is  at  once  parted  into  two  distinct  divisions,  the 
surface  water  and  the  ground  water:  the  former  courses 
more  or  less  swiftly,  generally  at  the  rate  of  a  mile  or 
more  an  hour,  in  the  light  of  day;  the  latter  enters  the 
interstices  of  the  earth,  slowly  descends  therein  to  a 
greater  or  less  depth,  and  finally,  journeying  perhaps  at 
the  rate  of  a  mile  a  year,  rejoins  the  surface  water,  escap- 
ing through  the  springs.  The  proportion  of  these  two 
classes,  the  surface  and  the  ground  water,  varies  greatly, 
and  an  intermixture  of  them  is  continually  going  on. 
Thus  on  the  surface  of  bare  rock  or  frozen  earth  all  the 
rain  may  go  aw^ay  without  entering  the  ground.     On  very 


172  OUTLINES    OP  THE  EARTH'S  HISTORY. 

sandy  fields  the  heaviest  rainfall  may  be  taken  up  by  the 
porous  earth,  so  that  no  streams  are  found.  On  such  sur- 
faces the  present  writer  has  observed  that  a  rainfall 
amounting  to  six  inches  in  depth  in  two  hours  produced 
no  streams  whatever.  We  shall  first  follow  the  history  of 
the  surface  water,  afterward  considering  the  work  which 
the  underground  movements  efi^ect. 

If  the  student  will  observe  what  takes  place  on  a  level 
ploughed  field — which,  after  all,  will  not  be  perfectly  level, 
for  all  fields  are  more  or  less  undulating — he  will  note  that, 
though  the  surface  may  have  been  smoothed  by  a  roller 
until  it  appears  like  a  floor,  the  first  rain,  where  the  fall 
takes  place  rapidly  enough  to  produce  surface  streams,  will 
create  a  series  of  little  channels  which  grow  larger  as  they 
conjoin,  the  whole  appearing  to  the  eye  like  a  very  de- 
tailed map,  or  rather  model,  of  a  river  system;  it  is,  indeed, 
such  a  system  in  miniature.  If  he  will  watch  the  process 
by  which  these  streamlet  beds  are  carved,  he  will  obtain 
a  tolerably  clear  idea  as  to  that  most  important  work  which 
the  greater  streams  do  in  carving  the  face  of  the  lands. 
The  water  is  no  sooner  gathered  into  a  sheet  than,  guided 
by  the  slightest  irregularities  which  it  encounters,  it  begins 
to  flow.  At  first  the  motion  is  so  slow  that  it  does  not 
disturb  its  bed,  but  at  some  points  in  the  bottom  of  the 
sheet  the  movement  soon  becomes  swift  enough  to  drag 
the  grains  of  sand  and  clay  from  their  adhesions,  bearing 
them  onward.  As  soon  as  this  beginning  of  a  channel  is 
formed  the  water  moves  more  swiftly  in  the  clearer  way; 
it  therefore  cuts  more  rapidly,  deepening  and  enlarging 
its  channel,  and  making  its  motion  yet  more  free.  The 
tiny  rills  join  the  greater,  all  their  channels  sway  to  and 
fro  as  directed  this  way  and  that  by  chance  irregularities, 
until  something  like  river  basins  are  carved  out,  those 
gentle  slopes  which  form  broad  valle)^s  where  the  carving 
has  been  due  to  the  wanderings  of  many  streams.  If  the 
field  be  large,  considerable  though  temporary  brooks  may 
be  created,  which  cut  channels  perhaps  a  foot  in  depth. 


THE  ATMOSPHERE.  173 

At  the  end  of  this  miniature  stream  system  we  always  find 
some  part  of  the  waste  which  has  been  carved  out.  If  the 
streamlet  discharges  into  a  pool,  we  find  the  tiny  repre- 
sentative of  deltas,  which  form  such  an  important  feature 
on  the  coast  line  where  large  rivers  enter  seas  or  lakes. 
Along  the  lines  of  the  stream  we  may  observe  here  and 
there  little  benches,  which  are  the  equivalent  in  all  save 
size  of  the  terraces  that  are  generally  to  be  observed  along 
the  greater  streams.  In  fact,  these  accidents  of  an  acre 
help  in  a  most  effective  way  the  student  to  understand 
the  greater  and  more  complicated  processes  of  continental 
erosion. 

A  normal  river — in  fact,  all  the  greater  streams  of  the 
earth — originates  in  high  country,  generally  in  a  region  of 
mountains.  Here,  because  of  the  elevation  of  the  region, 
the  streams  have  cut  deep  gorges  or  extensive  valleys,  all 
of  which  have  slopes  leading  steeply  downward  to  torrent 
beds.  Down  these  inclined  surfaces  the  particles  worn 
off  from  the  hard  rock  by  frost  and  by  chemical  decay 
gradually  work  their  way  until  they  attain  the  bed  of  the 
stream.  The  agents  which  assist  gravitation  in  bearing 
this  detritus  downward  are  many,  but  they  all  work  to- 
gether for  the  same  end.  The  stroke  of  the  raindrop  ac- 
complishes something,  though  but  little;  the  direct  wash- 
ing action  of  the  brooklets  which  form  during  times  of 
heavy  rain,  but  dry  out  at  the  close  of  the  storm,  do  a 
good  deal  of  the  work;  thawing  and  freezing  of  the  water 
contained  in  the  mass  of  detritus  help  the  movement,  for, 
although  the  thrust  is  in  both  directions,  it  is  most  effective 
downhill;  the  wedges  of  tree  roots,  which  often  penetrate 
between  and  under  the  stones,  and  there  expand  in  their 
process  of  growth,  likewise  assist  the  downward  motion. 
The  result  is  that  on  ordinary  mountain  slopes  the  layer 
of  fragments  constituting  the  rude  soil  is  often  creeping 
at  the  rate  of  from  some  inches  to  some  feet  a  year  toward 
the  torrent  bed.  If  there  be  cliffs  at  the  top  of  the  slope, 
as  is  often  the  case,  very  extensive  falls  of  rock  may  take 


174  OUTLINES  OF  THE  EARTH'S  HISTORY. 

place  from  it,  the  masses  descending  with  such  speed  that 
they  directly  attain  the  stream.  If  the  steeps  be  low  and 
the  rock  divided  into  vertical  joints,  especially  where  there 
is  a  soft  layer  at  the  base  of  the  steep,  detached  masses  from 
the  precipice  may  move  slowly  and  steadfastly  down  the 
slope,  so  little  disturbed  in  their  journey  that  trees  grow- 
ing upon  their  summits  may  continue  to  develop  for  the 
thousands  of  years  before  the  mass  enters  the  stream  bed. 

Although  the  fall  of  rocks  from  precipices  does  not 
often  take  place  in  a  conspicuously  large  way,  all  great 
mountain  regions  which  have  long  been  inhabited  by  man 
abound  in  traditions  and  histories  of  such  accidents.  With- 
in a  century  or  two  there  have  been  a  dozen  or  more  catas- 
trophes of  this  nature  in  the  inhabited  valleys  of  the  Alps. 
As  these  accidents  are  at  once  instructive  and  picturesque, 
it  is  well  to  note  certain  of  them  in  some  detail.  At 
Yvorgne,  a  little  parish  on  the  north  shore  of  the  Ehone, 
just  above  the  lake  of  Geneva,  tradition  tells  that  an  an- 
cient village  of  the  name  was  overwhelmed  by  the  fall  of 
a  great  cliff.  The  vast  debris  forming  the  steep  slope 
which  was  thus  produced  now  bears  famous  vineyards,  but 
the  vintners  fancy  that  they  from  time  to  time  hear  deep 
in  the  earth  the  ringing  of  the  bells  which  belonged  to 
the  overwhelmed  church.  In  1806  the  district  of  Goldau, 
just  north  of  Lake  Lucerne,  was  buried  beneath  the  ruins 
of  a  peak  which,  resting  upon  a  layer  of  clay,  slipped 
away  like  a  launching  ship  on  the  surface  of  the  soft  ma- 
terial. The  debris  overwhelmed  a  village  and  many  de- 
tached houses,  and  partly  filled  a  considerable  lake.  The 
wind  produced  by  this  vast  rush  of  falling  rock  was  so  great 
that  people  were  blown  away  by  it;  some,  indeed,  were 
killed  in  this  singular  manner. 

The  most  interesting  field  of  these  Swiss  mountain 
falls  is  a  high  mountain  valley  of  amphitheatrical  form, 
known  as  the  Diablerets,  or  the  devil's  own  district.  This 
great  circus,  which  lies  at  the  height  of  about  four  thou- 
sand feet  above  the  sea,  is  walled  around  on  its  northern 


THE  ATMOSPHERE.  175 

side  by  a  precipice,  above  which  rest,  or  rather  once  rested, 
a  number  of  mountain  peaks  of  great  bulk.  The  region 
has  long  been  valued  for  the  excellent  pasturage  which 
the  head  of  the  valley  affords.  Two  costly  roads,  indeed, 
have  been  built  into  it  to  afford  footpaths  for  the  flocks  and 
herds  and  their  keepers  in  the  summer  season.  Through 
this  human  experience  with  the  valley,  we  have  a  record 
of  what  has  gone  on  in  this  part  of  the  mountain  wilder- 
ness. Within  the  period  of  history  and  tradition,  three 
very  great  mountain  falls  have  occurred  in  this  field, 
each  having  made  its  memory  good  by  widespread  disaster 
which  it  brought  to  the  people  of  the  chalets.  The  last 
of  these  was  brought  about  by  the  fall  of  a  great  peak 
which  spread  itself  out  in  a  vast  field  of  ruins  in  the  valley 
below.  The  belt  of  destruction  was  about  half  a  mile 
wide  and  three  miles  long.  When  the  present  writer  last 
saw  it,  a  quarter  of  a  century  ago,  it  was  still  a  wilderness 
of  great  rocks,  but  here  and  there  the  process  of  their 
decay  was  giving  a  foothold  for  herbage,  and  in  a  few  cen- 
turies the  field  will  doubtless  be  so  verdure-clad  that  its 
story  will  not  be  told  on  its  face.  It  is  likely,  however, 
to  be  preserved  in  the  memory  of  the  people,  and  this 
through  a  singular  and  pathetic  tradition  which  has  grown 
up  about  the  place,  one  wdiich,  if  not  true,  comes  at  least 
among  the  legends  which  we  should  like  to  believe. 

As  told  the  present  writer  by  a  native  of  the  district, 
it  happened  when,  in  the  nighttime  the  mountain  came 
down,  the  herdsmen  and  their  cows  gathered  in  the  chalets 
— stout  buildings  which  are  prepared  to  resist  avalanches 
of  snow.  In  one  of  these,  which  was  protected  from  crush- 
ing by  the  position  of  the  stones  which  covered  it,  a  solitary 
herdsman  found  himself  alive  in  his  unharmed  dwelling. 
With  him  in  the  darkness  were  the  cows,  a  store  of  food 
and  water,  and  his  provisions  for  the  long  summer  season. 
With  nothing  but  hope  to  animate  him,  he  set  to  work 
burrowing  upward  among  the  rocks,  storing  the  debris 
in  the  room  of  the  chalet.    He  toiled  for  some  months,  but 


176  OUTLINES   OF  THE  EARTH'S  HISTORY. 

finally  emerged  to  the  light  of  day,  blanched  by  his  long 
imprisonment  in  the  darkness,  but  with  the  strength  to 
bear  him  to  his  home.  In  place  of  the  expected  warm 
welcome,  the  unhappy  man  found  himself  received  as  a 
ghost.  He  was  exorcised  by  the  priest  and  driven  away 
to  the  distance.  It  was  only  when  long  afterward  his  path 
of  escape  was  discovered  that  his  history  became  known. 

Returning  to  the  account  of  the  debris  which  descends 
at  varied  speed  into  the  torrents,  we  find  that  when  the 
detritus  encounters  the  action  of  these  vigorous  streams 
it  is  rapidly  ground  to  pieces  while  it  is  pushed  down  the 
steep  channels  to  the  lower  country.  Where  the  stones 
are  of  such  size  that  the  stream  can  urge  them  on,  they 
move  rapidly;  at  least  in  times  when  the  torrent  is  raging. 
They  beat  over  each  other  and  against  the  firm-set  rocks; 
the  more  they  wear,  the  smaller  they  become,  and  the  more 
readily  they  are  urged  forward.  Where  the  masses  are  too 
large  to  be  stirred  by  the  violent  current,  they  lie  unmoved 
until  the  pounding  of  the  rolling  stones  reduces  them  to 
the  proportions  where  they  may  join  the  great  procession. 
Ordinarily  those  who  visit  mountains  behold  their  torrents 
only  in  their  shrunken  state,  when  the  waters  stir  no 
stones,  and  fail  even  to  bear  a  charge  of  mud,  all  detach- 
able materials  having  been  swept  away  when  the  streams 
course  with  more  vigour.  In  storm  seasons  the  conditions 
are  quite  otherwise;  then  the  swollen  torrents,  their  waters 
filled  with  clay  and  sand,  bear  with  them  great  quantities 
of  boulders,  the  collisions  of  which  are  audible  above  the 
muffled  roar  of  the  waters,  attesting  the  very  great  energy 
of  the  action. 

When  the  waste  on  a  mountain  slope  lies  at  a  steep 
angle,  particularly  where  the  accumulation  is  due  to  the 
action  of  ancient  glaciers,  it  not  infrequently  happens  that 
when  the  ground  is  softened  with  frost  great  masses  of 
the  material  rush  down  the  slope  in  the  manner  of  land- 
slides. The  observer  readily  notes  that  in  many  mountain 
regions,  as,  for  instance,  in  the  White  Mountains  of  New 


THE  ATMOSPHEKE.  lY7 

Hampshire,  the  steep  slopes  are  often  seamed  by  the  paths 
of  these  great  landslides.  Their  movement,  indeed,  is 
often  begun  by  sliding  snow,  which  gives  an  impulse  to 
the  rocks  and  earth  which  it  encounters  in  its  descent.  At 
a  place  known  as  the  Wylie  Notch,  in  the  White  Moun- 
tains, in  the  early  part  of  this  century,  a  family  of  that 
name  was  buried  beneath  a  mass  of  glacial  waste  which 
had  hung  on  the  mountain  slope  from  the  ancient  days 
until  a  heavy  rain,  following  on  a  period  of  thaw,  impelled 
the  mass  down  the  slope.  Although  there  have  been  few 
such  catastrophes  noted  in  this  country,  it  is  because  our 
mountains  have  not  been  much  dwelt  in.  As  they  become 
thickly  inhabited  as  the  Alps  are,  men  are  sure  to  suffer 
from  these  accidents. 

As  the  volume  of  a  mountain  torrent  increases  through 
the  junction  of  many  tributaries,  the  energy  of  its  moving 
waters  becomes  sufficient  to  sweep  away  the  fragments 
which  come  to  its  bed.  Before  this  stage  is  attained  the 
stream  rarely  touches  the  solid  under  rock  of  the  moun- 
tain, the  base  of  the  current  resting  upon  the  larger  loose 
stones  which  it  was  unable  to  stir.  In  this  pebble-paved 
section,  because  the  stream  could  not  attack  the  foundation 
rock,  we  find  no  gorges — in  fact,  the  whole  of  this  upper 
section  of  the  torrent  system  is  peculiarly  conditioned  by 
the  fact  that  the  streams  are  dealing  not  with  bed-rock, 
but  with  boulders  or  smaller  loose  fragments.  If  they  cut 
a  little  channel,  the  materials  from  either  side  slip  the 
faster,  and  soon  repave  the  bed.  But  when  the  streams 
have  by  a  junction  gained  strength,  and  can  keep  their 
beds  clear,  they  soon  carve  down  a  gorge  through  which 
they  descend  from  the  upper  mountain  realm  to  the  larger 
valleys,  where  their  conjoined  waters  take  on  a  riverlike 
aspect.  It  should  be  noted  here  that  the  cutting  power 
of  the  water  moving  in  the  torrent  or  in  the  wave,  the 
capacity  it  has  for  abrading  rock,  resides  altogether  in  the 
bits  of  stone  or  cutting  tools  with  which  it  is  armed.  Pure 
water,  because  of  its  fluidity,  may  move  over  or  against 


178  OUTLINE^   OF  THE  EARTH'S  HISTORY. 

firm-set  stones  for  ages  without  wearing  them;  but  in  pro- 
portion as  it  moves  rocky  particles  of  any  size,  the  larger 
they  are,  the  more  effective  the  work,  it  wears  the  rock 
over  which  it  flows.  A  capital  instance  of  this  may  he 
found  where  a  stream  from  a  hose  is  used  in  washing  win- 
dows. If  the  water  be  pure,  there  is  no  effect  upon  the 
glass;  but  if  it  be  turbid,  containing  bits  of  sand,  in  a  little 
while  the  surface  will  appear  cloudy  from  the  multitude  of 
fine  scratches  which  the  hard  bits  impelled  by  the  water 
have  inflicted  upon  it.  A  somewhat  similar  case  occurs 
where  the  wind  bears  sand  against  window  panes  or  a 
bottle  which  has  long  lain  on  the  shore.  The  glass  will 
soon  be  deeply  carved  by  the  action,  assuming  the  appear- 
ance which  we  term  "  ground.^'  This  principle  is  made 
use  of  in  the  arts.  Glass  vessels  or  sheets  are  prepared 
for  carving  by  pasting  paper  cut  into  figures  on  their  sur- 
faces. The  material  is  then  exposed  to  a  jet  of  air  or 
steam-impelling  sand  grains;  in  a  short  time  all  the  sur- 
face which  has  not  been  protected  by  paper  has  its  polish 
destroyed  and  is  no  longer  translucent. 

The  passage  from  the  torrent  to  the  river,  though  not 
in  a  geographical  way  distinct,  is  indicated  to  the  observant 
eye  by  a  simple  feature — namely,  the  appearance  of  allu- 
vial terraces,  those  more  or  less  level  heaps  of  water-borne 
debris  which  accumulate  along  the  banks  of  rivers,  which, 
indeed,  constitute  the  difference  between  those  streams 
and  torrents.  Where  the  mountain  waters  move  swiftly, 
they  manage  to  bear  onward  the  waste  which  they  receive. 
Even  where  the  blocks  of  stone  cling  in  the  bed,  it  is  only 
a  short  time  before  they  are  again  set  in  motion  or  ground 
to  pieces.  If  by  chance  the  detritus  accumulates  rapidly, 
the  slope  is  steepened  and  the  w^ork  of  the  torrent  made 
more  efficient.  As  the  torrent  comes  toward  the  base  of 
the  mountains,  where  it  neither  finds  nor  can  create  steep 
slopes  over  which  to  flow,  its  speed  necessarily  diminishes. 
With  each  reduction  in  this  feature  its  carrying  power  very 
rapidly  diminishes.    Thus  water  flowing  at  the  rate  of  ten 


THE  ATMOSPHERE.  179 

miles  an  hour  can  urge  stones  four  times  the  mass  that  it 
can  move  when  its  speed  is  reduced  to  half  that  rate.  The 
result  is  that  on  the  lowlands,  with  their  relatively  gentle 
slopes,  the  combined  torrents,  despite  the  increase  in  the 
volume  of  the  stream  arising  from  their  confluence,  have 
to  lay  down  a  large  part  of  their  load  of  detritus. 

If  we  watch  where  a  torrent  enters  a  mountain  river, 
we  observe  that  the  main  stream  in  a  way  sorts  over  the 
waste  contributed  to  it,  bearing  on  only  those  portions 
which  its  rate  of  flow  will  permit  it  to  carry,  leaving  the 
remainder  to  be  built  into  the  bank  in  the  form  of  a  rude 
terrace.  This  accumulation  may  not  extend  far  below 
the  point  where  the  torrent  which  imported  the  debris 
joins  the  main  stream;  a  little  farther  down,  however,  we 
are  sure  to  find  another  such  junction  and  a  second  accu- 
mulation of  terrace  material.  As  these  contributions  in- 
crease, the  terrace  accumulations  soon  become  continuous, 
lying  on  one  side  or  the  other  of  the  river,  sometimes  bor- 
dering both  banks  of  the  stream.  In  general,  it  can  be 
said  that  so  long  as  the  rate  of  fall  of  the  torrent  exceeds 
one  hundred  feet  to  the  mile  it  does  not  usually  exhibit 
these  shelves  of  detritus.  Below  that  rate  of  descent  they 
are  apt  to  be  formed.  Much,  however,  depends  upon  the 
amount  of  detritus  which  the  stream  bears  and  the  coarse- 
ness of  it;  moreover,  where  the  water  goes  through  a  gorge 
in  the  manner  of  a  flume  with  steep  rocky  sides,  it  can 
urge  a  larger  amount  before  it  than  when  it  traverses  a 
wide  valley,  through  which  it  passes,  it  may  be,  in  a  wind- 
ing way. 

At  first  sight  it  may  seem  rather  a  fine  distinction  to 
separate  torrents  from  rivers  by  the  presence  or  absence  of 
terraces.  As  we  follow  down  the  stream,  however,  and  study 
its  action  in  relation  to  these  terraces,  and  the  peculiar  his- 
tory of  the  detritus  of  which  they  are  composed,  we  per- 
ceive that  these  latter  accumulations  are  very  important 
features.  Beginning  at  first  with  small  and  imperfect 
alluvial  plains,  the  river,  as  it  descends  toward  the  sea. 


180  OUTLINES  OF  THE  EARTH'S  HISTORY. 

gaining  in  store  of  water  and  in  the  amount  of  deiris  which 
comes  with  that  water  from  the  hills,  while  the  rate  of  fall 
and  consequent  speed  of  the  current  are  diminished,  soon 
comes  to  a  stage  where  it  is  engaged  in  an  endless  strug- 
gle with  the  terrace  materials.  In  times  of  flood,  the  walls 
of  the  terraces  compel  the  tide  to  flow  over  the  tops  of 
these  accumulations.  Owing  to  the  relative  thinness  of 
the  water  beyond  the  bed,  and  to  the  growth  of  vegetation 
there,  the  current  moves  more  slowly,  and  therefore  lays 
down  a  considerable  deposit  of  the  silt  and  sand  which  it 
contains.  This  may  result  during  a  single  flood  in  lift- 
ing the  level  of  the  terrace  by  some  inches  in  height,  still 
further  serving  to  restrict  the  channel.  Along  the  banks 
of  the  Mississippi  and  other  large  rivers  the  most  of  this 
detritus  falls  near  the  stream;  a  little  of  it  penetrates  to 
the  farther  side  of  the  plains,  which  often  have  a  width 
of  ten  miles  or  more.  The  result  is  -that  a  broad  elevation 
is  constructed,  a  sort  of  natural  mole  or  levee,  in  a  meas- 
ure damming  the  flood  waters,  which  can  now  only  enter 
the  "  back  swamps  "  through  the  channels  of  the  tributary 
streams.  Each  of  these  back  swamps  normally  discharges 
into  the  main  stream  through  a  little  river  of  its  own,  along 
the  banks  of  which  the  natural  levees  do  not  develop. 

We  have,  now  to  note  a  curious  swinging  movement  of 
rivers  which  was  first  well  observed  by  the  skilful  engi- 
neers of  British  India.  This  movement  can  best  be  illus- 
trated by  its  effects.  If  on  any  river  which  winds  through 
alluvial  plains  a  jetty  is  so  constructed  as  to  deflect  the 
stream  at  any  point,  the  course  which  it  follows  will  be 
altered  during  its  subsequent  flow,  it  may  be,  for  the  dis- 
tance of  hundreds  of  miles.  It  will  be  perceived  that  in  its 
movements  a  river  normally  strikes  flrst  against  one  shore 
and  then  against  the  other.  Its  w^ater  in  a  general  way 
moves  as  does  a  billiard  ball  when  it  flies  from  one  cushion 
to  another.  It  is  true  that  in  a  torrent  we  have  the  same 
conditions  of  motion;  but  there  the  banks  are  either  of 
bard  rock  or^  if  of  detritus^  they  are  continually  moving 


THE  ATMOSPHERE.  181 

into  the  stream  in  the  manner  before  described.  In  the 
case  of  the  river,  however,  its  points  of  collision  are  often 
on  soft  banks,  which  are  readily  undermined  by  the  wash- 
ing action  of  the  stream.  In  the  ordinary  course  of  events, 
the  river  beginning,  we  may  imagine,  with  a  straight  chan- 
nel, had  its  current  deflected  by  some  obstacle,  it  may  be 
even  by  the  slight  pressure  of  a  tributary  stream,  is  driven 
against  one  bank;  thence  it  rebounds  and  strikes  the  other. 
At  each  point  of  impinge  it  cuts  the  alluvium  away.  It 
can  bear  on  only  a  small  portion  of  that  which  it  thus 
obtains;  the  greater  part  of  the  material  is  deposited  on 
the  opposite  side  of  the  stream,  but  a  little  lower  down, 
where  it  makes  a  shallow.  On  these  shallows  water-loving 
plants  and  even  certain  trees,  such  as  the  willows  and  pop- 
lars, find  a  foothold.  When  the  stream  rises,  the  sediment 
settles  in  this  tangle,  and  soon  extends  the  alluvial  plain 
from  the  neighbouring  bank,  or  in  rarer  cases  the  river 
comes  to  flow  on  either  side  of  an  island  of  its  own  con- 
struction. The  natural  result  of  this  billiard-ball  move- 
ment of  the  waters  is  that  the  path  of  the  stream  is  sinu- 
ous. The  less  its  rate  of  fall  and  the  greater  the  amount 
of  silt  it  obtains  from  its  tributaries,  the  more  winding 
its  course  becomes.  This  gain  in  those  parts  of  the  river's 
curvings  where  deposition  tends  to  take  place  may  be 
accelerated  by  tree-planting.  Thus  a  skilful  owner  of  a 
tract  of  land  on  the  south  bank  of  the  Ohio  Eiver,  by 
assiduously  planting  willow  trees  on  the  front  of  his  prop- 
erty, gained  in  the  course  of  thirty  years  more  than  an 
acre  in  the  width  of  his  arable  land.  When  told  by  the 
present  writer  that  he  was  robbing  his  neighbours  on  the 
other  side  of  the  stream,  he  claimed  that  their  ignorance 
of  the  laws  of  river  motion  was  sufficient  evidence  that  they 
did  not  deserve  to  own  land. 

In  the  primitive  state  of  a  country  the  water-loving 
plants,  particularly  the  trees  which  flourish  in  excessively 
humid  conditions,  generally  make  a  certain  defence  against 
these  incursions  of  the  streams.     But  when  a  river  has 


182 


OUTLINES  OF  THE  EARTH'S  HISTORY. 


gained  an  opening  in  the  bank  it  can^  during  a  flood,  ex- 
tend its  width  often  to  the  distance  of  hundreds  of  feet. 
During  the  inundations  of  the  Mississippi  the  river  may 
at  times  be  seen  to  eat  away  acres  of  land  in  a  single  day 
along  one  of  the  outcurves  of  its  banks.  The  undermined 
forests  falling  into  the  flood  join  the  great  procession  of 
drift  timber,  composed  of  trees  which  have  been  similarly 
uprooted,  which  occupies  the  middle  part  of  the  stream. 
This  driftwood  belt  often  has  a  width  of  three  or  four  hun- 
dred feet,  the  entangled  stems  and  branches  making  it 
difficult  for  a  boat  to  pass  from  one  side  of  the  river  to 
the  other. 


Fia.  11. — Oxbows  and  cut-off.    Showing  the  changes  in  the  course  of 
a  river  in  its  alluvial  plain. 


When  the  curves  of  a  river  have  been  developed  to  a 
certain  point  (see  Fig.  11),  when  they  have  attained  what 


THE  ATMOSPHERE.  183 

is  called  the  "  oxbow "  form,  it  often  happens  that  the 
stream  breaks  through  the  isthmus  which  connects  one 
of  the  peninsulas  with  the  mainland.  Where,  as  is  not  in- 
frequently the  case,  the  bend  has  a  length  of  ten  miles  or 
more,  the  water  just  above  and  below  the  new-made  open- 
ing is  apt  to  differ  in  height  by  some  feet.  Plunging  down 
the  declivity,  the  stream,  flowing  with  great  velocity,  soon 
enlarges  the  channel  so  that  its  whole  tide  may  take  the 
easier  way.  When  this  result  is  accomplished,  the  old 
curve  is  deserted,  sand  bars  are  formed  across  their  mouths, 
which  may  gradually  grow  to  broad  alluvial  plains,  so  that 
the  long-surviving,  crescent-shaped  lake,  the  remnant  of 
the  river  bed,  may  be  seen  far  from  the  present  course  of 
the  ever-changing  stream.  Gradually  the  accumulations 
of  vegetable  matter  and  the  silt  brought  in  by  floods  efface 
this  moat  or  oxbow  cut-off,  as  it  is  so  commonly  termed. 

As  soon  as  the  river  breaks  through  the  neck  of  a 
peninsula  in  the  manner  above  described,  the  current  of 
the  stream  becomes  much  swifter  for  many  miles  below 
and  above  the  opening.  Slowly,  however,  the  slopes  are 
rearranged  throughout  its  whole  course,  yet  for  a  time 
the  stream  near  the  scat  of  the  change  becomes  straighter 
than  before,  and  this  for  the  reason  that  its  swifter  cur- 
rent is  better  able  to  dispose  of  the  debris  which  is  supplied 
to  it.  The  effect  of  a  change  in  the  current  produced  by 
such  new  channels  as  we  have  described  as  forming  across 
the  isthmuses  of  bends  is  to  perturb  the  course  of  the 
stream  in  all  its  subsequent  downward  length.  Thus  an 
oxbow  cut-off  formed  near  the  junction  of  the  Ohio  and 
Mississippi  may  tend  more  or  less  to  alter  the  swings  of 
the  Mississippi  all  the  way  to  the  Gulf  of  Mexico. 

Although  the  swayings  of  the  streams  to  and  fro  in 
their  alluvial  plains  will  give  the  reader  some  idea  as  to 
the  struggle  which  the  greater  rivers  have  with  the  dchris 
which  is  committed  to  them,  the  full  measure  of  the  work 
and  its  consequences  can  only  be  appreciated  by  those 
who  have  studied  the  phenomena  on  the  ground.  A  river 
13 


184:  OUTLINES  OP  THE  EARTH'S  HISTORY. 

sucli  as  the  Mississippi  is  endlessly  endeavouring  to  bear 
its  burden  to  the  sea.  If  its  slope  were  a  uniform  in- 
clined plane,  the  task  might  readily  be  accomplished;  but 
in  this,  as  in  almost  all  other  large  water  ways,  the  slope 
of  the  bed  is  ever  diminishing  with  its  onward  course.  The 
same  water  which  in  the  mountain  torrent  of  the  Appa- 
lachians or  Cordilleras  rolled  along  stones  several  feet  in 
diameter  down  slopes  of  a  hundred  feet  or  more  to  the 
mile  can  in  the  lower  reaches  of  the  stream  move  no  peb- 
bles which  are  more  than  one  fourth  of  an  inch  in  diameter 
over  slopes  which  descend  on  the  average  about  half  a  foot 
in  a  mile.  Thus  at  every  stage  from  the  torrent  to  the  sea 
the  detritus  has  from  time  to  time  to  rest  within  the  allu- 
vial banks,  there  awaiting  the  decay  which  slowly  comes, 
and  which  may  bring  it  to  the  state  where  it  may  be  dis- 
solved in  the  water,  or  divided  into  fragments  so  small  that 
the  stream  may  bear  them  on.  A  computation  which  the 
present  writer  has  made  shows  that,  on  the  average,  it  re- 
quires about  forty  thousand  years  for  a  particle  of  stone 
to  make  its  way  down  the  Mississippi  to  the  sea  after  it  has 
been  detached  from  its  original  bed.  Of  course,  some  bits 
may  make  the  journey  straightforwardly;  others  may  re- 
quire a  far  greater  time  to  accomplish  the  course  which 
the  water  itself  makes  at  most  in  a  few  weeks.  This  long 
delay  in  the  journey  of  the  detritus — a  delay  caused  by  its 
frequent  rests  in  the  alluvial  plain — ^brings  about  impor- 
tant consequences  which  we  will  now  consider. 

As  an  alluvial  plain  is  constructed,  we  generally  find 
at  the  base  pebbly  material  which  fell  to  the  bottom  in 
the  current  of  the  main  stream  as  the  shores  grew  outward. 
Above  this  level* we  find  the  deposits  laid  down  by  the 
flood  waters  containing  no  pebbles,  and  this  for  the  reason 
that  those  weightier  bits  remained  in  the  stream  bed  when 
the  tide  flowed  over  the  plain.  As  the  alluvial  deposit  is 
laid  down,  a  good  deal  of  vegetable  matter  was  built  into 
it.  Generally  this  has  decayed  and  disappeared.  On  the 
surface  of  the  plain  there  has  always  been  growing  abun- 


THE  ATMOSPHERE.  185 

dant  vegetation,  the  remains  of  which  decayed  on  the  sur- 
face in  the  manner  which  we  may  observe  at  the  present 
day.  This  decomposing  vegetable  matter  within  and  upon 
the  porous  alluvial  material  produces  large  quantities  of 
carbonic  acid,  a  gas  which  readily  enters  the  rain  water, 
and  gives  it  a  peculiar  power  of  breaking  up  rock  matter. 
Acting  on  the  debris,  this  gas-charged  water  rapidly  brings 
about  a  decay  of  the  fragments.  Much  of  the  material 
passes  at  once  into  solution  in  this  water,  and  drains  away 
through  the  multitudinous  springs  which  border  the  river. 
As  this  matter  is  completely  dissolved,  as  is  sugar  in 
water,  it  goes  straight  away  to  the  sea  without  ever  again 
entering  the  alluvium.  In  many,  if  not  most,  cases  this 
dissolving  work  which  is  going  on  in  alluvial  terraces  is 
sufficient  to  render  a  large  part  of  the  materials  which 
they  contain  into  the  state  where  it  disappears  in  an  un- 
seen manner;  thus  while  the  annual  floods  are  constantly 
laying  down  accumulations  on  the  surface  of  these  plains, 
the  springs  are  bearing  it  away  from  below. 

In  this  way,  through  the  decomposition  which  takes 
place  in  them,  all  those  river  terraces  w^here  much  vegeta- 
ble matter  is  mingled  with  the  mineral  substances,  become 
laboratories  in  which  substances  are  brought  into  solution 
and  committed  to  the  seas.  We  find  in  the  water  of  the 
ocean  a  great  array  of  dissolved  mineral  substances;  it, 
indeed,  seems  probable  that  the  sea  water  contains  some 
share,  though  usually  small,  of  all  the  materials  which 
rivers  encounter  in  their  journey  over  and  under  the  lands. 
As  the  waters  of  the  sea  obtain  but  little  of  this  dissolved 
matter  along  the  coast,  it  seems  likely  that  the  greater  share 
of  it  is  brought  into  the  state  of  solution  in  the  natural 
laboratories  of  the  alluvial  plains. 

Here  and  there  along  the  sides  of  the  valleys  in  which 
the  rivers  flow  we  commonly  find  the  remains  of  ancient 
plains  lying  at  more  or  less  considerable  heights  above 
the  level  of  the  streams.  Generally  these  deposits,  which 
from  their  form  are  called  terraces,  represent  the  stages  of 


186  OUTLINES  OF  THE  EARTH'S  HISTORY. 

down-wearing  by  which  the  stream  has  carved  out  its  way 
through  the  rocks.  The  greater  part  of  these  ancient  allu- 
vial plains  has  been  removed  through  the  ceaseless  swing- 
ing of  the  stream  to  and  fro  in  the  valley  which  it  has  ex- 
cavated. 

In  all  the  states  of  alluvial  plains,  whether  they  be 
the  fertile  deposits  near  the  level  of  the  streams  which 
built  them,  or  the  poorer  and  ruder  surfaced  higher  ter- 
races, they  have  a  great  value  to  mankind.  Men  early 
learned  that  these  lands  were  of  singularly  uniform  good- 
ness for  agricultural  use.  They  are  so  light  that  they  were 
easily  delved  with  the  ancient  pointed  sticks  or  stone  hoes, 
or  turned  by  the  olden  wooden  plough.  They  not  only 
give  a  rich  return  when  first  subjugated,  but,  owing  to  the 
depth  of  the  soil  and  the  frequency  with  which  they  are 
visited  by  fertilizing  inundations,  they  yield  rich  harvests 
without  fertilizing  for  thousands  of  years.  It  is  therefore 
not  surprising  that  we  find  the  peoples  who  depended  upon 
tillage  for  subsistence  first  developed  on  the  great  river 
plains.  There,  indeed,  were  laid  the  foundations  of  our 
higher  civilization;  there  alone  could  the  state  which  de- 
mands of  its  citizens  fixed  abodes  and  continuous  labour 
take  rise.  In  the  conditions  which  these  fields  of  abun- 
dance afforded,  dense  populations  were  possible,  and  all 
the  arts  which  lead  toward  culture  were  greatly  favoured. 
Thus  it  is  that  the  civilization  of  China,  India,  Persia,  and 
Egypt,  the  beginnings  of  man's  higher  development,  began 
near  the  mouths  of  the  great  river  valleys.  These  fields 
were,  moreover,  most  favourably  placed  for  the  institution 
of  commerce,  in  that  the  arts  of  navigation,  originating 
in  the  sheltered  reaches  of  the  streams,  readily  found  its 
way  through  the  estuaries  to  the  open  sea. 

Passing  down  the  reaches  of  a  great  river  as  it  ap- 
proaches the  sea,  we  find  that  the  alluvial  plains  usually 
widen  and  become  lower.  At  length  we  attain  a  point 
where  the  fiood  waters  cover  the  surface  for  so  large  a  part 
of  the  year  that  the  ground  is  swampy  and  untiUable  un- 


THE  ATMOSPHERE.  187 

less  it  is  artificially  and  at  great  expense  of  labour  won  to 
agriculture  in  the  manner  in  which  this  task  has  been 
effected  in  the  lower  portion  of  the  Rhine  Valley.  Still 
farther  toward  the  sea,  the  plain  gradually  dips  downward 
until  it  passes  below  the  level  of  the  waters.  Through  this 
mud-flat  section  the  stream  continues  to  cut  channels,  but 
with  the  ever-progressive  slowing  of  its  motion  the  burden 
of  fine  mud  which  it  carries  drops  to  the  bottom,  and 
constantly  closes  the  paths  through  which  the  water  es- 
capes. Every  few  years  they  tend  to  break  a  new  way  on 
one  side  or  the  other  of  their  former  path.  Some  of  the 
greatest  engineering  work  done  in  modern  times  has  been 
accomplished  by  the  engineers  engaged  in  controlling  the 
exits  of  large  rivers  to  the  sea.  The  outbreak  of  the  Yellow 
River  in  1887,  in  which  the  stream,  hindered  by  its  own 
accumulations,  forced  a  new  path  across  its  alluvial  plains, 
destroyed  a  vast  deal  of  life  and  property,  and  made  the 
new  exit  seventy  miles  from  the  path  which  it  abandoned. 

Below  the  surface  of  the  open  water  the  alluvial  de- 
posits spread  out  into  a  broad  fan,  which  slopes  gradually 
to  a  point  where,  in  the  manner  of  the  continental  shelf, 
the  bottom  descends  steeply  into  deep  water. 

It  is  the  custom  of  naturalists  to  divide  the  lower  sec- 
tion of  river  deposits — that  part  of  the  accumulation  which 
is  near  the  sea — from  the  other  alluvial  plains,  terming 
the  lower  portion  the  delta.  The  word  originally  came 
into  use  to  describe  that  part  of  the  alluvium  accumulated 
by  the  Nile  near  its  mouth,  which  forms  a  fertile  territory 
shaped  somewhat  like  the  fourth  letter  of  the  Greek  alpha- 
bet. Although  the  definition  is  good  in  the  Egyptian  in- 
stance, and  has  a  certain  use  elsewhere,  we  best  regard  all 
the  detritus  in  a  river  valley  which  is  in  the  state  of  repose 
along  the  stream  to  its  utmost  branches  as  forming  one 
great  whole.  It  is,  indeed,  one  of  the  most  united  of  the 
large  features  which  the  earth  exhibits.  The  student 
should  consider  it  as  a  continuous  inclined  plane  of  dimin- 
ishing slope,  extending  from  the  base  of  the  torrents  to 


188  OUTLINES  OF  THE  EARTH'S  HISTORY. 

the  sea,  and  of  course  ramifying  into  the  several  branches 
of  the  river  system.  He  should  further  bear  in  mind  the 
fact  that  it  is  a  vast  laboratory  where  rock  material  is 
brought  into  the  soluble  state  for  delivery  to  the  seas. 

The  diversity  in  the  form  of  river  valleys  is  exceedingly 
great.  Almost  all  the  variety  of  the  landscape  is  due  to 
this  impress  of  water  action  which  has  operated  on  the 
surface  in  past  ages.  When  first  elevated  above  the  sea, 
the  surface  of  the  land  is  but  little  varied;  at  this  stage 
in  the  development  the  rivers  have  but  shallow  valleys, 
which  generally  cut  rather  straight  away  over  the  plain 
toward  the  sea.  It  is  when  the  surface  has  been  uplifted 
to  a  considerable  height,  and  especially  when,  as  is  usually 
the  case,  this  uplifting  action  has  been  associated  with 
mountain-building,  that  valleys  take  on  their  accented  and 
picturesque  form.  The  reason  for  this  is  easily  perceived: 
it  lies  in  the  fact  that  the  rocks  over  which  the  stream  flows 
are  guided  in  the  cutting  which  they  effect  by  the  diversi- 
ties of  hardness  in  the  strata  that  they  encounter.  The 
work  which  it  does  is  performed  by  the  hard  substances 
that  are  impelled  by  the  current,  principally  by  the  sand 
and  pebbles.  These  materials,  driven  along  by  the  stream, 
become  eroding  tools  of  very  considerable  energy.  As  will 
be  seen  when  we  shortly  come  to  describe  waterfalls,  the 
potholes  formed  at  those  points  afford  excellent  evidence 
as  to  the  capacity  of  stream-impelled  bits  of  stone  to  cut 
away  the  firmest  bed  rocks.  Naturally  the  ease  with  which 
this  carving  w^ork  is  done  is  proportionate  to  the  energy  of 
the  currents,  and  also  to  the  relative  hardness  of  the  mov- 
ing bits  and  the  rocks  over  which  they  are  driven. 

So  long  as  the  rocks  lie  horizontally  in  their  natural 
construction  attitude  the  course  of  the  stream  is  not  much 
influenced  by  the  variations  in  hardness  which  the  bed 
exhibits.  Where  the  strata  are  very  firm  there  is  likely  to 
be  a  narrow  gorge,  the  steeps  of  which  rise  on  either  side 
with  but  slight  alluvial  plains;  where  the  beds  are  soft 
the  valley  widens,  perhaps  again  to  contract  where  in  the 


THE  ATMOSPHERE.  189 

course  of  its  descent  it  encounters  another  hard  layer. 
Where,  however,  the  beds  have  been  subjected  to  mountain- 
building,  and  have  been  thrown  into  very  varied  attitudes 
by  folding  and  faulting,  the  stream  now  here  and  now  there 
encounters  beds  which  either  restrain  its  flow  or  give  it 
freedom.  The  stream  is  then  forced  to  cut  its  way  accord- 
ing to  the  positions  of  the  various  underlying  strata.  This 
effect  upon  its  course  is  not  only  due  to  the  peculiarities 
of  uplifted  rocks,  but  to  manifold  accidents  of  other  na- 
ture: veins  and  dikes,  which  often  interlace  the  beds  with 
harder  or  softer  partitions  than  the  country  rock;  local 
hardenings  in  the  materials,  due  to  crystallization  and 
other  chemical  processes,  often  create  indescribable  varia- 
tions which  are  more  or  less  completely  expressed  in  the 
path  of  the  stream. 

When  a  land  has  been  newly  elevated  above  the  sea 
there  is  often — we  may  say,  indeed,  generally — a  very  great 
difference  between  the  height  of  its  head  waters  and  the 
ocean  level.  In  this  condition  of  a  country  the  rivers  have 
what  we  may  call  a  new  aspect;  their  valleys  are  commonly 
narrow  and  rather  steep,  waterfalls  are  apt  to  abound,  and 
the  alluvial  terraces  are  relatively  small  in  extent.  Stage 
by  stage  the  torrents  cut  deeper;  the  waste  which  they 
make  embarrasses  the  course  of  the  lower  waters,  where  no 
great  amount  of  down-cutting  is  possible  for  the  reason 
that  the  bed  of  the  stream  is  near  sea  level.  At  the  same 
time  the  alluvial  materials,  building  out  to  sea,  thus  dimin- 
ish the  slope  of  the  stream.  .In  the  extreme  old  age  of  the 
river  system  the  mountains  are  eaten  down  so  that  the  tor- 
rent section  disappears,  and  the  stream  becomes  of  some- 
thing like  a  uniform  slope;  the  higher  alluvial  plains  grad- 
ually waste  away,  until  in  the  end  the  valley  has  no  salient 
features.  At  this  stage  in  the  process,  or  even  before  it  is 
attained,  the  valley  is  likely  to  be  submerged  beneath  the 
sea,  where  it  is  buried  beneath  the  deposits  formed  on  the 
floor;  or  a  further  uplift  of  the  land  may  occur  with  the 
result  that  the  stream  is  rejuvenated;  or  once  more  en- 


190  OUTLINES  OF  THE  EARTH'S  HISTORY. 

dowed  with  the  power  to  create  torrents,  build  alluvial 
plains,  and  do  the  other  interesting  work  of  a  normal 
river. 

It  rarely,  if  ever,  happens  that  a  river  valley  attains 
old  age  before  it  has  sunk  beneath  the  sea  or  been  refreshed 
by  further  upliftings.  In  the  unstable  conditions  of  the 
continents,  one  or  the  other  of  these  processes,  sometimes 
in  different  places  both  together,  is  apt  to  be  going  on. 
Thus  if  we  take  the  case  of  the  Mississippi  and  its  prin- 
cipal tributaries,  the  Ohio  and  Missouri,  we  find  that  for 
many  geological  ages  the  mountains  about  their  sources 
have  frequently,  if  not  constantly,  grown  upward,  so  that 
their  torrent  sections,  though  they  have  worn  down  tens 
of  thousands  of  feet,  are  still  high  above  the  sea  level,  per- 
haps on  the  average  as  high  as  they  have  ever  been.  At 
the  same  time  the  slight  up-and-down  swayings  of  the 
shore  lands,  amounting  in  general  to  less  than  five  hundred 
feet,  have  greatly  affected  the  channels  of  the  main  river 
and  its  tributaries  in  their  lower  parts.  Not  long  ago  the 
Mississippi  between  Cairo  and  the  Gulf  flowed  in  a  rather 
steep-sided  valley  probably  some  hundreds  of  feet  in  depth, 
which  had  a  width  of  many  miles.  Then  at  the  close  of  the 
last  Glacial  period  the  region  sank  down  so  that  the  sea 
flooded  the  valley  to  a  point  above  the  present  junction  of 
the  Ohio  Kiver  with  the  main  stream.  Since  then  alluvial 
plains  have  filled  this  estuary  to  even  beyond  the  original 
mouth.  In  many  other  of  our  Southern  rivers,  as  along  the 
shore  from  the  Mississippi  to  the  Hudson,  the  streams  have 
'  not  brought  in  enough  detritus  to  fill  their  drowned  valleys, 
which  have  now  the  name  of  bays,  of  which  the  Delaware 
and  Chesapeake  on  the  Atlantic  coast,  and  Mobile  Bay  on 
the  Gulf  of  Mexico,  are  good  examples.  The  failure  of 
Chesapeake  and  Delaware  Bays  to  fill  with  debris  in  the 
measure  exhibited  by  the  more  southern  valleys  is  due  to 
the  fact  that  the  streams  which  flow  into  them  to  a  great 
extent  drain  from  a  region  thickly  covered  .with  glacial 
waste,  a  mass  which  holds  the  flood  waters,  yielding  the 


THE  ATMOSPHERE.  191 

supply  but  slowly  to  the  torrents,  which  there  have  but 
a  slight  cutting  power. 

In  our  sketch  of  river  valleys  no  attention  has  been 
given  to  the  phenomena  of  waterfalls,  those  accidents  of 
the  flow  which,  as  we  have  noted,  are  particularly  apt  to 
characterize  rivers  which  have  not  yet  cut  down  to  near 
the  sea  level.  Where  the  normal  uniform  descent  which  is 
characteristic  of  a  river's  bed  is  interrupted  by  a  sudden 
steep,  the  fact  always  indicates  the  occurrence  of  one  of 
a  number  of  geological  actions.  The  commonest  cause  of 
waterfalls  is  due  to  a  sudden  change  in  the  character  of 
horizontal  or  at  least  nearly  level  beds  over  which  the 
stream  may  flow.  Where  after  coursing  for  a  distance 
over  a  hard  layer  the  stream  comes  to  its  edge  and  drops 
on  a  soft  or  easily  eroded  stratum,  it  will  cut  this  latter  bed 
away,  and  create  a  more  or  less  characteristic  waterfall. 
Tumblhig  down  the  face  of  the  hard  layer,  the  stream  ac- 
quires velocity;  the  debris  which  it  conveys  is  hurled 
against  the  bottom,  and  therefore  cuts  powerfully,  while 
before,  being  only  rubbed  over  the  stone  as  it  moved  along, 
it  cut  but  slightly.  Masses  of  ice  have  the  same  effect  as 
stones.'  Bits  dropping  from  the  ledge  are  often  swept  round 
and  round  by  the  eddies,  so  that  they  excavate  an  opening 
which  prevents  their  chance  escape.  In  these  confined 
spaces  they  work  like  augers,  boring  a  deep,  well-like  cav- 
ity. As  the  bits  of  stone  wear  out  they  are  replaced  by 
others,  which  fall  in  from  above.  Working  in  this  way, 
the  fragments  often  develop  regular  well-like  depressions, 
the  cavities  of  which  work  back  under  the  cliffs,  and  by 
the  undermining  process  deprive  the  face  of  the  wall  of  its 
support,  so  that  it  tumbles  in  ruin  to  the  base,  there  to 
supply  more  material  for  the  potholing  action. 

Waterfalls  of  the  type  above  described  are  by  far  the 
commonest  of  those  which  occur  out  of  the  torrent  dis- 
tricts of  a  great  river  system.  That  of  Niagara  is  an  ex- 
cellent specimen  of  the  type,  which,  though  rarely  mani- 
fested in  an3^thing  like  the  dignity  of  the  great  fall,  is 


192  OUTLINES  OF  THE  EARTH'S  HISTORY. 

plentifully  shown  throughout  the  Mississippi  Valley  and 
the  basin  of  the  Great  Lakes.  Within  a  hundred  miles  of 
Niagara  there  are  at  least  a  hundred  small  waterfalls  of 
the  same  type.  Probably  three  quarters  of  all  the  larger 
accidents  of  this  nature  are  due  to  the  conditions  of  a  hard 
bed  overlying  softer  strata. 

Falls  are  also  produced  in  very  many  instances  by 
dikes  which  cross  the  stream.  So,  too,  though  rarely, 
only  one  striking  instance  being  known,  an  ancient  coral 
reef  which  has  become  buried  in  strata  may  afford  rock 
of  such  hardness  that  when  the  river  comes  to  cross  it  it 
forms  a  cascade,  as  at  the  Falls  of  the  Ohio,  at  Louisville, 
Ky.  It  is  a  characteristic  of  all  other  falls,  except  those 
first  mentioned,  that  they  rarely  plunge  with  a  clean  down- 
ward leap  over  the  face  of  a  precipice  which  recedes  at  its 
base,  but  move  downward  over  an  irregular  sloping  surface. 

In  the  torrent  district  of  rivers  waterfalls  are  commonly 
very  numerous,  and  are  generally  due  to  the  varying  hard- 
ness in  the  rocks  which  the  streams  encounter.  Here, 
where  the  cutting  action  is  going  on  with  great  rapidity, 
slight  differences  in  the  resistance  which  the  rocks  make 
to  the  work  will  lead  to  great  variations  in  the  form  of  the 
bed  over  which  they  flow,  while  on  the  more  gently  sloping 
bottoms  of  the  rivers,  where  the  debris  moves  slowly,  such 
variations  would  be  unimportant  in  their  effect.  When 
the  torrents  escape  into  the  main  river  valleys,  in  regions 
where  the  great  streams  have  cut  deep  gorges,  they  often 
descend  from  a  great  vertical  height,  forming  wonderful 
waterfalls,  such  as  those  which  occur  in  the  famous  Lauter- 
brunnen  Valley  of  Switzerland  or  in  that  of  the  Yosemite 
in  California.  This  group  of  cascades  is  peculiar  in  that 
the  steep  of  the  fall  is  made  not  by  the  stream  itself,  but 
by  the  action  of  a  greater  river  or  of  a  glacier  which  may 
have  some  time  taken  its  place. 

Waterfalls  have  an  economic  as  well  as  a  picturesque 
interest  in  that  they  afford  sources  of  power  which  may 
be  a  very  great  advantage  to  manufacturers.    Thus  along 


THE  ATMOSPHERE.  193 

the  Atlantic  coast  the  streams  which  come  from  the  Appa- 
lachian highlands,  and  which  have  hardly  escaped  from 
their  torrent  section  before  they  attain  the  sea,  afford  nu- 
merous cataracts  which  have  been  developed  so  that  they 
afford  a  vast  amount  of  power.  Between  the  James  on  the 
south  and  the  Ste.  Croix  on  the  north  more  than  a  hundred 
of  these  Appalachian  rivers  have  been  turned  to  economic 
use.  The  industrial  arts  of  this  part  of  the  country  depend 
much  upon  them  for  the  power  which  drives  their  machin- 
ery. The  whole  of  the  United  States,  because  of  the  con- 
siderable size  of  its  rivers  and  their  relatively  rapid  fall, 
is  richly  endowed  with  this  source  of  energy,  which,  origi- 
nating in  the  sun's  heat  and  conveyed  through  the  rain, 
may  be  made  to  serve  the  needs  of  man.  In  view  of  the 
fact  that  recent  inventions  have  made  it  possible  to  con- 
vert this  energy  of  falling  water  into  the  form  of  elec- 
tricity, which  may  be  conveyed  to  great  distances,  it  seems 
likely  that  our  rivers  will  in  the  future  be  a  great  source 
of  national  wealth. 

We  must  turn  again  to  river  valleys,  there  to  trace 
certain  actions  less  evident  than  those  already  noted,  but 
of  great  importance  in  determining  these  features  of  the 
land.  First,  we  have  to  note  that  in  the  valley  or  region 
drained  by  a  river  there  is  another  degrading  or  down- 
wearing  action  than  that  which  is  accomplished  by  the 
direct  work  of  the  visible  stream.  All  over  such  a  valley 
the  underground  waters,  soaking  through  the  soil  and 
penetrating  through  the  underlying  rock,  are  constantly 
removing  a  portion  of  the  mineral  matter  which  they  take 
into  solution  and  bear  away  to  the  sea.  In  this  way.  de- 
prived of  a  part  of  their  substance,  the  rocks  are  continual- 
ly settling  down  by  underwear  throughout  the  whole  basin, 
while  they  are  locally  being  cut  down  by  the  action  of  the 
stream.  Hence  in  part  it  comes  about  that  in  a  river  basin 
we  find  two  contrasted  features — the  general  and  often 
slight  slope  of  a  country  toward  the  main  stream  and  its 
greater  tributaries,  and  the  sharp  indentation  of  the  gorge 


194  OUTLINES  OF  THE  EARTH'S  HISTORY. 

in  which  the  streams  flow,  these  latter  caused  by  the  imme- 
diate and  recent  action  of  the  streams. 

If  now  the  reader  will  conceive  himself  standing  at  any 
point  in  a  river  basin,  preferably  beyond  the  realms  of  the 
torrents,  he  may  with  the  guidance  of  the  facts  previously 
noted,  with  a  little  use  of  the  imagination,  behold  the  vast 
perceptive  which  the  history  of  the  river  valley  may  unfold 
to  him.  He  stands  on  the  surface  of  the  soil,  that  debris 
of  the  rocks  which  is  just  entering  on  its  way  to  the  ocean. 
In  the  same  region  ten  thousand  years  ago  he  would  have 
stood  upon  a  surface  from  one  to  ten  feet  higher  than  the 
present  soil  covering.  A  million  years  ago  his  station  would 
have  been  perhaps  five  hundred  feet  higher  than  the  sur- 
face. Ten  million  years  in  the  past,  a  period  less  than  the 
lifetime  of  certain  rivers,  such  as  the  French  Broad  Eiver 
in  North  Carolina,  the  soil  was  probably  five  thousand  feet 
or  more  above  its  present  plane.  There  are,  indeed,  cases 
where  river  valleys  appear  to  have  worked  down  without 
interruption  from  the  subsidence  of  the  land  beneath  the 
sea  to  the  depth  of  at  least  two  miles.  Looking  upward 
through  the  space  which  the  rocks  once  occupied,  we  can 
conceive  the  action  of  the  forces  in  their  harmonious  co- 
operation which  have  brought  the  surface  slowly  down- 
ward. We  can  imagine  the  ceaseless  corrosion  due  to  the 
ground  water,  bringing  about  a  constant  though  slow 
descent  of  the  whole  surface.  Again  and  again  the  streams, 
swinging  to  and  fro  under  the  guidance  of  the  under- 
lying rock,  or  from  the  obstacles  which  the  debris  they 
carried  imposed  upon  them,  have  crossed  the  surface.  Now 
and  then  perhaps  the  wearing  was  intensified  by  glacial 
action,  for  an  ice  sheet  often  cuts  with  a  speed  many  times 
as  great  as  that  which  fluid  water  can  accomplish.  On  the 
whole,  this  exercise  of  the  constructive  imagination  in  con- 
ceiving the  history  of  a  river  valley  is  one  of  the  most 
enlarging  tasks  which  the  geologist  can  undertake. 

Where  in  a  river  valley  there  are  many  lateral  streams, 
and  especially  where  the  process  of  solution  carried  on  by 


THE  ATMOSPHERE.  195 

the  underground  waters  is  most  effective,  as  compared  with 
erosive  work  done  in  the  bed  of  the  main  river,  we  com- 
monly find  the  valley  sloping  gently  toward  its  centre,  the 
rivers  having  but  slight  steeps  near  their  banks.  On  the 
other  hand,  where,  as  occasionally  happens,  a  considerable 
stream  fed  by  the  rain  and  snow  fall  in  its  torrent  section 
courses  for  a  great  distance  over  high,  arid  plains,  on  which 
the  ground  water  and  the  tributaries  do  but  little  work, 
the  basin  may  slope  with  very  slight  declivity  to  the  river 
margins,  and  there  descend  to  great  depths,  forming  very 
deep  gorges,  of  which  the  Colorado  Canon  is  the  most  per- 
fect type.  As  instances  of  these  contrasted  conditions,  we 
may  take,  on  the  one  hand,  the  upper  Mississippi,  where 
the  grades  toward  the  main  stream  are  gentle  and  the 
valley  gorge  but  slightly  exhibited;  on  the  other,  the  above- 
mentioned  Colorado,  which  bears  a  great  tide  of  waters 
drawn  from  the  high  and  relatively  rainy  region  of  the 
Rocky  Mountains  across  the  vast  plateau  lying  in  an  almost 
rainless  country.  In  this  section  nearly  all  the  down-wear- 
ing has  been  brought  about  in  the  direct  path  of  the 
stream,  which  has  worn  the  elevated  plain  into  a  deep  gorge 
during  the  slow  uprising  of  the  table-land  to  its  present 
height.  In  this  way  a  defile  nearly  a  mile  in  depth  has 
been  created  in  a  prevailingly  rather  flat  country.  This 
gorge  has  embranchments  where  the  few  great  tributaries 
have  done  like  work,  but,  on  the  whole,  this  river  flows  in 
an  almost  unbroken  channel,  the  excavation  of  which  has 
been  due  to  its  swift,  pebble-bearing  waters. 

The  tendency  of  a  newly  formed  river  is  to  cut  a  more 
or  less  distinct  caiion.  As  the  basin  becomes  ancient,  this 
element  of  the  gorge  tends  to  disappear,  the  reason  for  this 
being  that,  while  the  river  bed  is  high  above  the  sea,  the 
current  is  swift  and  the  down-cutting  rapid,  while  the  slow 
subsidence  of  the  country  on  either  side — a  process  w^hich 
goes  on  at  a  uniform  rate — causes  the  surface  of  that  region 
to  be  left  behind  in  the  race  for  the  sea  level.  As  the 
stream  bed  comes  nearer  the  sea  level  its  rate  of  descent 


IOC  OUTLINES  OF  THE  EARTH'S  HISTORY. 

is  diminished,  and  so  the  outlying  country  gradually  over- 
takes it. 

In  regions  where  the  winters  are  very  cold  the  effect 
of  ice  on  the  development  of  the  stream  beds  both  in  the 
torrent  and  river  sections  of  the  valley  is  important.  This 
work  is  accomplished  in  several  diverse  ways.  In  the  first 
place,  where  the  stream  is  clear  and  the  current  does  not 
flow  too  swiftly,  the  stones  on  the  bottom  radiate  their 
heat  through  the  water,  and  thus  form  ice  on  their  surfaces, 
which  may  attain  considerable  thickness.  As  ice  is  con- 
siderably lighter  than  water,  the  effect  is  often  to  lift  up 
the  stones  of  the  bed  if  they  be  not  too  large;  when  thus 
detached  from  the  bottom,  they  are  easily  floated  down 
stream  until  the  ice  melts  away.  The  ice  which  forms  on 
the  surface  of  the  water  likewise  imprisons  the  pebbles 
along  the  banks,  and  during  the  subsequent  thaw  may 
carry  them  hundreds  of  miles  toward  the  sea.  It  seems 
likely,  from  certain  observations  made  by  the  writer,  that 
considerable  stones  may  thus  be  carried  from  the  Alle- 
ghany River  to  the  main  Mississippi. 

Perhaps  the  most  important  effect  of  ice  on  river  chan- 
nels is  accomplished  when  in  a  time  of  flood  the  ice  field 
which  covered  the  stream,  perhaps  to  the  depth  of  some 
feet,  is  broken  up  into  vast  floes,  which  drift  downward 
with  the  current.  When,  as  on  the  Ohio,  these  fields  some- 
times have  the  area  of  several  hundred  acres,  they  often 
collide  with  the  shores,  especially  where  the  stream  makes 
a  sharp  bend.  Urged  by  their  momentum,  these  ice  floes 
pack  into  the  semblance  of  a  dam,  which  may  have  a  thick- 
ness of  twenty,  thirty,  or  even  fifty  feet.  Beginning  on 
the  shore,  where  the  collision  takes  place,  the  dam  may 
swiftly  develop  clear  across  the  stream,  so  that  in  a  few 
minutes  the  way  of  the  waters  is  completely  blocked.  The 
on-coming  ice  shoots  up  upon  the  accumulation,  increases 
its  height,  and  extends  it  up  stream,  so  that  in  an  hour 
the  mass  completely  bars  the  current.  The  waters  then 
heap  up  until  they  break  their  way  over  the  obstacle,  wash- 


THE  ATMOSPHERE.  I97 

ing  its  top  away,  until  the  whole  is  light  enough  to  be 
forced  down  the  stream,  where,  by  the  friction  it  encoun- 
ters on  the  bottom  and  sides  of  the  channel,  it  is  broken  to 
pieces.  It  is  easy  to  see  that  such  moving  dams  of  ice  may 
sweep  the  bed  of  a  river  as  with  a  great  broom. 

Sometimes  where  the  gorges  do  not  form  a  stationary 
dam  large  cakes  of  ice  become  turned  on  edge  and  pack 
together  so  that  they  roll  down  the  stream  like  great  wheels, 
grinding  the  bed  rock  as  they  go. 

In  high  northern  countries,  as  in  Siberia,  the  rivers, 
even  the  deepest,  often  become  so  far  frozen  that  their  chan- 
nels are  entirely  obstructed.  Where,  as  in  the  case  of  these 
Siberian  rivers,  the  flow  is  from  south  to  north,  it  often 
happens  that  the  spring  thaw  sets  in  before  the  more  north- 
ern beds  of  the  main  stream  are  released  from  their  bond- 
age of  frost.  In  this  case  the  inundations  have  to  find  new 
paths  on  either  side  of  the  obstructed  way.  The  result  is 
a  type  of  valleys  characterized  by  very  irregular  and 
changeable  stream  beds,  the  rivers  having  no  chance  to 
organize  themselves  into  the  shapely  curves  which  they 
ordinarily  follow. 

The  supply  which  finds  its  way  to  a  river  is  composed, 
as  has  been  already  incidentally  noted,  in  part  of  the  water 
which  courses  underground  for  a  greater  or  less  distance 
before  it  emerges  to  the  surface,  and  in  part  of  that  which 
moves  directly  over  the  ground.  These  two  shares  of  water 
have  somewhat  different  histories.  On  the  share  of  these 
two  depends  the  stability  of  the  flow.  Where,  as  in  New 
England  and  other  glaciated  countries,  the  surface  of  the 
earth  is  covered  with  a  thick  layer  of  sand  and  gravel, 
which,  except  when  frozen,  readily  admits  the  water;  the 
rainfall  is  to  a  very  great  extent  absorbed  by  the  earth,  and 
only  yielded  slowly  to  the  streams.  In  these  cases  floods 
are  rare  and  of  no  great  destructive  power.  Again,  where 
also  the  river  basin  is  covered  by  a  dense  mantle  of  forests, 
the  ground  beneath  which  is  coated,  as  is  the  case  in  prime- 
val woods,  with  a  layer  of  decomposing  vegetation  a  foot  or 


4 

198  OUTLINES  OF  THE  EARTH'S  HISTORY. 

more  in  depth,  this  spongy  mass  retains  the  water  even 
more  effectively  than  the  open-textured  glacial  deposits 
above  referred  to.  When  the  woods,  however,  are  removed 
from  such  an  area,  the  rain  may  descend  to  the  streams 
almost  as  speedily  as  it  finds  its  way  to  the  gutters  from 
the  house  roofs.  It  thus  comes  about  that  all  regions, 
when  reduced  to  tillage,  and  where  the  rainfall  is  enough 
to  maintain  a  good  agriculture,  are,  except  when  they  have 
a  coating  of  glacial  waste,  exceedingly  liable  to  destructive 
inundations. 

Unhappily,  the  risk  of  river  floods  is  peculiarly  great  in 
all  the  regions  of  the  United  States  lying  much  to  the  east 
of  the  Rocky  Mountains,  except  in  the  basin  of  the  Great 
Lakes  and  in  the  district  of  New  England,  where  the  preva- 
lence of  glacial  sands  and  gravels  affords  the  protection 
which  we  have  noted.  Throughout  this  region  the  rainfall 
is  heavy,  and  the  larger  part  of  it  is  apt  to  come  after  the 
ground  has  become  deeply  snow-covered.  The  result  is  a 
succession  of  devastating  floods  which  already  are  very 
damaging  to  the  works  of  man,  and  promise  to  become 
more  destructive  as  time  goes  on.  More  than  in  any  other 
country,  we  need  the  protection  which  forests  can  give  us 
against  these  disastrous  outgoings  of  our  streams. 

Lakes. 

In  considering  the  journey  of  water  from  the  hilltops 
to  the  sea,  we  should  take  some  account  of  those  pauses 
which  it  makes  on  its  way  when  for  a  time  it  falls  into  the 
basin  of  a  lake.  These  arrests  in  the  downward  motion 
of  water,  which  we  term  lakes,  are  exceedingly  numerous; 
their  proper  discussion  would,  indeed,  require  a  consider- 
able volume.  We  shall  here  note  only  the  more  important 
of  their  features,  those  which  are  of  interest  to  the  gen- 
eral student. 

The  first  and  most  noteworthy  difference  in  lakes  is 
that  which  separates  the  group  of  dead  seas  from  the  living 


THE  ATMOSPHERE.  199 

basins  of  fresh  water.  When  a  stream  attains  a  place  where 
its  waters  have  to  expand  into  the  lakelike  form,  the  cur- 
rent moves  in  a  slow  manner,  and  the  broad  surface  ex- 
posed to  the  air  permits  a  large  amount  of  evaporation.  If 
the  basin  be  large  in  proportion  to  the  amount  of  the  in- 
current  water,  this  evaporation  may  exceed  the  supply, 
and  produce  a  sea  with  no  outlet,  such  as  we  find  in  the 
Dead  Sea  of  Judea,  in  that  at  Salt  Lake,  Utah,  and  in  a 
host  of  other  less  important  basins.  If  the  rate  of  evapora- 
tion be  yet  greater  in  proportion  to  the  flow,  the  lake  may 
altogether  dry  away,  and  the  river  be  evaporated  before 
it  attains  the  basin  where  it  might  accumulate.  In  that 
case  the  river  is  said  to  sink,  but,  in  place  of  sinking  into 
the  earth,  its  waters  really  rise  into  the  air.  Many  such 
sinks  occur  in  the  central  portion  of  the  Rocky  Mountain 
district.  It  is  important  to  note  that  the  process  of  evap- 
oration w^e  are  describing  takes  place  in  the  case  of  all  lakes, 
though  only  here  and  there  is  the  air  so  dry  that  the 
evaporation  prevents  the  basin  from  overflowing  at  the 
lowest  point  on  its  rim,  forming  a  river  which  goes  thence 
to  the  sea.  Even  in  the  case  of  the  Great  Lakes  of  North 
America  a  considerable  part  of  the  water  which  flows  into 
them  does  not  go  to  the  St.  Lawrence  and  thence  to  the  sea. 
As  long  as  the  lake  finds  an  outlet  to  the  sea  its  waters 
contain  but  little  more  dissolved  mineral  matter  than  that 
we  find  in  the  rivers.  But  because  all  water  which  has 
been  in  contact  with  the  earth  has  some  dissolved  mineral 
substances,  while  that  which  goes  away  by  evaporation  is 
pure  water,  a  lake  without  an  outlet  gradually  becomes 
so  charged  with  these  materials  that  it  can  hold  no  more 
in  solution,  but  proceeds  to  lay  them  down  in  deposits  of 
that  compound  substance  which  from  its  principal  ingredi- 
ent we  name  salt.  The  water  of  dead  seas,  because  of  the 
additional  weight  of  the  substances  which  it  holds,  is  ex- 
traordinarily buoyant.  The  swimmer  notes  a  difference 
in  this  regard  in  the  waters  of  rivers  and  fresh-water  lakes 
and  those  of  the  sea,  due  to  this  same  cause.  But  in  those 
14 


200  OUTLINES  OF  THE  EARTH'S  HISTORY. 

of  dead  seas,  saturated  with  saline  materials,  the  human 
body  can  not  sink  as  it  does  in  the  ordinary  conditions  of 
immersion.  It  is  easy  to  understand  how  the  salt  deposits 
which  are  mined  in  many  parts  of  the  world  have  gener- 
ally, if  not  in  all  cases,  been  formed  in  such  dead  seas.* 

It  is  an  interesting  fact  that  almost  all  the  known  dead 
seas  have  in  recent  geological  times  been  living  lakes — that 
is,  they  poured  over  their  brims.  In  the  Cordilleras  from 
the  line  between  Canada  and  the  United  States  to  central 
Mexico  there  are  several  of  these  basins.  All  of  those  which 
have  been  studied  show  by  their  old  shore  lines  that  they 
were  once  brimful,  and  have  only  shrunk  away  in  modern 
times.  These  conditions  point  to  the  conclusion  that  the 
rainfall  in  different  regions  varies  greatly  in  the  course  of 
the  geologic  ages.  Further  confirmation  of  this  is  found 
in  the  fact  that  very  great  salt  deposits  exist  on  the  coast 
of  Louisiana  and  in  northern  Europe — regions  in  which 
the  rainfall  is  now  so  great  in  proportion  to  the  evapora- 
tion that  dead  seas  are  impossible. 

Turning  now  to  the  question  of  how  lake  basins  are 
formed,  we  note  a  great  variety  in  the  conditions  which 
may  bring  about  their  construction.  The  greatest  agent, 
or  at  least  that  which  operates  in  the  construction  of  the 
largest  basins,  are  the  irregular  movements  of  the  earth, 
due  to  the  mountain-building  forces.  Where  this  w^ork 
goes  on  on  a  large  scale,  basin-shaped  depressions  are  in- 
evitably formed.  If  all  those  which  have  existed  remained, 
the  large  part  of  the  lands  would  be  covered  by  them.  In 
most  cases,  however,  the  cutting  action  of  the  streams  has 
been  sufficient  to  bring  the  drainage  channels  down  to  the 
bottom  of  the  trough,  while  the  influx  of  sediments  has 
served  to  further  the  work  by  filling  up  the  cavities.    Thus 


*  In  some  relatively  rare  cases  salt  deposits  are  formed  in  lagoons 
along  the  shores  of  arid  lands,  where  the  sea  occasionally  breaks  over 
the  beach  into  the  basin,  affording  waters  which  are  evaporated, 
leaving  their  salt  behind  them. 


THE  ATMOSPHERE.  201 

at  the  close  of  the  Cretaceous  period  there  was  a  chain  of 
lakes  extending  along  the  eastern  base  of  the  Rocky  Moun- 
tains, constituting  fresh-water  seas  probably  as  large  as 
the  so-called  Great  Lakes  of  North  America.  But  the  rivers, 
by  cutting  down  and  tilling  up,  have  long  since  obliterated 
these  water  areas.  In  other  cases  the  tiltings  of  the  con- 
tinent, which  sometimes  oppose  the  flow  of  the  streams, 
may  for  a  time  convert  the  upper  part  of  a  river  basin 
which  originally  sloped  gently  toward  the  sea  into  a  cav- 
ity. Several  cases  of  this  description  occurred  in  New 
England  in  the  closing  stages  of  the  Glacial  period,  when 
the  ground  rose  up  to  the  northward. 

We  have  already  noted  the  fact  that  the  basin  of  a  dead 
sea  becomes  in  course  of  time  the  seat  of  extensive  salt 
deposits.  These  may,  indeed,  attain  a  thickness  of  many 
hundred  feet.  If  now  in  the  later  history  of  the  country 
the  tract  of  land  with  the  salt  beneath  it  were  traversed 
by  a  stream,  its  underground  watery  may  dissolve  out  the 
salt  and  in  a  way  restore  the  basin  to  its  original  unfilled 
condition,  though  in  the  second  state  that  of  a  living  lake. 
It  seems  very  probable  that  a  portion  at  least  of  the  areas 
of  Lakes  Ontario,  Erie,  and  Huron  may  be  due  to  this 
removal  of  ancient  salt  deposits,  remains  of  which  lie 
buried  in  the  earth  in  the  region  bordering  these  basins. 

By  far  the  commonest  cause  of  lake  basins  is  found  in 
the  irregularities  of  the  surface  which  are  produced  by 
the  occupation  of  the  country  by  glaciers.  When  these 
great  sheets  of  ice  lie  over  a  land,  they  are  in  motion  down 
the  slopes  on  which  they  rest;  they  wear  the  bed  rocks  in  a 
vigorous  manner,  cutting  them  down  in  proportion  to  their 
hardness.  As  these  rocks  generally  vary  in  the  resistance 
which  they  oppose  to  the  ice,  the  result  is  that  when  the 
glacier  passes  away  the  surface  no  longer  exhibits  the  con- 
tinued down  slope  which  the  rivers  develop,  but  is  warped 
in  a  very  complicated  way.  These  depressions  afford  nat- 
ural basins  in  which  lakes  gather;  they  may  vary  in  extent 
from  a  few  square  feet  to  many  square  miles.    When  a  gla- 


202  OUTLINES  OF  THE  EARTH'S  HISTORY. 

cier  occupies  a  country,  the  melting  ice  deposits  on  the 
surface  of  the  earth  a  vast  quantity  of  rocky  debris,  which 
Avas  contained  in  its  mass.  This  detritus  is  irregularly 
accumulated;  in  part  it  is  disposed  in  the  form  of  moraines 
or  rude  mounds  made  at  the  margin  of  the  glacier,  in  part 
as  an  irregular  sheet,  now  thick,  now  thin,  which  covers 
the  whole  of  the  field  over  which  the  ice  lay.  The  result 
of  this  action  is  the  formation  of  innumerable  pools,  which 
continue  to  exist  until  the  streams  have  cut  channels 
through  which  their  waters  may  drain  away,  or  the  basins 
have  become  filled  with  detritus  imported  from  the  sur- 
rounding country  or  by  peat  accumulations  which  the 
plants  form  in  such  places. 

Doubtless  more  than  nine  tenths  of  all  the  lake  basins, 
especially  those  of  small  size,  which  exist  in  the  world  are 
due  to  irregularities  of  the  land  surface  which  are  brought 
about  by  glacial  action.  Although  the  greater  part  of  these 
small  basins  have  been  obliterated  since  the  ice  left  this 
country,  the  number  still  remaining  of  sufficient  size  to 
be  marked  on  a  good  map  is  inconceivably  great.  In  North 
America  alone  there  are  probably  over  a  hundred  and  fifty 
thousand  of  these  glacial  lakes,  although  by  far  the  greater 
part  of  those  which  existed  when  the  glacial  sheet  dis- 
appeared have  been  obliterated. 

Yet  another  interesting  group  of  fresh-water  lakes,  or 
rather  we  should  call  them  lakelets  from  their  small  size, 
owes  its  origin  to  the  curious  underground  excavations  or 
caverns  which  are  formed  in  limestone  countries.  The 
water  enters  these  caverns  through  what  are  termed  "  sink 
holes  " — basins  in  the  surface  which  slope  gently  toward  a 
central  opening  through  which  the  water  flows  into  the 
depths  below.  The  cups  of  the  sink  holes  rarely  exceed 
half  a  mile  in  diameter,  and  are  usually  much  smaller. 
Their  basins  have  been  excavated  by  the  solvent  and  cut- 
ting actions  of  the  rain  water  which  gathers  in  them  to  be 
discharged  into  the  cavern  below.  It  often  Happens  that 
after  a  sink  hole  is  formed  some  slight  accident  closes  the 


THE  ATMOSPHERE.  203 

downward-leading  shaft,  so  that  the  basin  holds  water; 
thus  in  parts  of  the  United  States  there  are  thousands  of 
these  nearly  circular  pools,  which  in  certain  districts,  as  in 
southern  Kentucky,  serve  to  vary  the  landscape  in  much 
the  same  manner  as  the  glacial  lakes  of  more  northern 
countries. 

Some  of  the  most  beautiful  lakes  in  the  world,  though 
none  more  than  a  few  miles  in  diameter,  occupy  the  craters 
of  extinct  volcanoes.  When  for  a  time,  or  permanentl3%  a 
volcano  ceases  to  do  its  appointed  w^ork  of  pouring  forth 
steam  and  molten  rock  from  the  depths  of  the  earth,  the 
pit  in  the  centre  of  the  cone  gathers  the  rain  water,  form- 
ing a  deep  circular  lake,  which  is  walled  round  by  the  pre- 
cipitous faces  of  the  crater.  If  the  volcano  reawakens,  the 
water  which  blocks  its  passage  may  be  blown  out  in  a  mo- 
ment, the  discharge  spreading  in  some  cases  to  a  great 
distance  from  the  cone,  to  be  accumulated  again  w^hen 
the  vent  ceases  to  be  open.  The  most  beautiful  of  these 
volcanic  lakes  are  to  be  found  in  the  region  to  the  north 
and  south  of  Rome.  The  original  seat  of  the  Latin  state 
was  on  the  shores  of  one  of  these  crater  pools,  south  of  the 
Eternal  City.  Lago  Bolsena,  which  lies  to  the  northward, 
and  is  one  of  the  largest  known  basins  of  this  nature, 
having  a  diameter  of  about  eight  miles,  is  a  crater  lake. 
The  volcanic  cone  to  which  it  belongs,  though  low,  is  of 
great  size,  showing  that  in  its  time  of  activity,  which  did 
not  endure  very  long,  this  crater  was  the  seat  of  mighty 
ejections.  The  noblest  specimen  of  this  group  of  basins 
is  found  in  Crater  Lake,  Oregon,  now  contained  in  one  of 
the  national  parks  of  the  United  States. 

Inclosed  bodies  of  water  are  formed  in  other  ways  than 
those  described;  the  list  above  given  includes  all  the  im- 
portant classes  of  action  which  produce  these  interesting 
features.  We  should  now  note  the  fact  that,  unlike  the 
seas,  the  lakes  are  to  be  regarded  as  temporary  features 
in  the  physiography  of  the  land.  One  and  all,  they  en- 
dure for  but  brief  geologic  time,  for  the  reason  that  the 


204r  OUTLINES  OF  THE  EARTH'S  HISTORY. 

streams  work  to  destroy  them  by  filling  tliem  with  sedi- 
ment and  by  carving  out  channels  through  Avhich  their 
waters  drain  away.  The  nature  of  this  action  can  well  be 
conceived  by  considering  what  will  take  place  in  the  course 
of  time  in  the  Great  Lakes  of  North  America.  As  Niagara 
Falls  cut  back  at  the  average  rate  of  several  feet  a  year,  it 
will  be  but  a  brief  geologic  period  before  they  begin  to 
lower  the  waters  of  Lake  Erie.  It  is  very  probable,  indeed, 
that  in  twenty  thousand  years  the  waters  of  that  basin 
will  be  to  a  great  extent  drained  away.  When  this  occurs, 
another  fall  or  rapid  will  be  produced  in  the  channel  which 
leads  from  Lake  Huron  to  Lake  Erie.  This  in  turn  will 
go  through  its  process  of  retreat  until  the  former  expanse 
of  waters  disappears.  The  action  will  then  be  continued 
at  the  outlets  of  Lakes  Michigan  and  Superior,  and  in  time, 
but  for  the  interposition  of  some  actions  which  recreate 
these  basins,  their  floors  will  be  converted  into  dry  land. 

It  is  interesting  to  note  that  lakes  owe  in  a  manner  the 
preservation  of  their  basins  to  an  action  which  they  bring 
about  on  the  waters  that  flow  into  them.  These  rivers 
or  torrents  commonly  convey  great  quantities  of  sediment, 
which  serve  to  rasp  their  beds  and  thus  to  lower  their  chan- 
nels. In  all  but  the  smaller  lakelets  these  turbid  waters  lay 
down  all  their  sediment  before  they  attain  the  outlet  of 
the  basin.  Thus  they  flow  away  over  the  rim  rock  in  a  per- 
fectly pure  state — a  state  in  which,  as  we  have  noted  be- 
fore, water  has  no  capacity  for  abrading  firm  rock.  Thus 
where  the  Niagara  River  passes  from  Lake  Erie  its  clean 
water  hardly  affects  the  stone  over  which  it  flows.  It  only 
begins  to  do  cutting  work  where  it  plunges  down  the  preci- 
pice of  the  Falls  and  sets  in  motion  the  fragments  which 
are  constantly  falling  from  that  rocky  face.  These  Falls 
could  not  have  begun  as  they  did  on  the  margin  of  Lake 
Ontario  except  for  the  fact  that  when  the  Niagara  River 
began  to  flow,  as  in  relatively  modern  times,  it  found  an 
old  precipice  on  the  margin  of  Lake  Ontario,  formed  by 
the  waves  of  the  lake,  down  which  the  waters  fell,  and 


THE  ATMOSPHERE.  206 

where  they  ohtained  cutting  tools  with  which  to  undermine 
the  steep  which  forms  the  Falls. 

Many  great  lakes,  particularly  those  which  we  have 
just  been  considering,  have  repeatedly  changed  their  out- 
lets, according  as  the  surface  of  the  land  on  which  they 
lie  has  swayed  up  and  down  in  various  directions,  or  as 
glacial  sheets  have  barred  or  unbarred  the  original  outlets 
of  the  basins.  Thus  in  the  Laurentian  Lakes  above  On- 
tario the  geologist  finds  evidence  that  the  drainage  lines 
have  again  and  again  been  changed.  For  a  time  during 
the  Glacial  period,  when  Lake  Ontario  and  the  valley  of 
the  St.  Lawrence  was  possessed  by  the  ice,  the  discharge 
was  southward  into  the  upper  Mississippi  or  the  Ohio.  At 
a  later  stage  channels  were  formed  leading  from  Georgian 
Bay  to  the  eastern  part  of  Ontario.  Yet  later,  when  the 
last-named  lake  was  bared,  an  ice  dam  appears  to  have 
remained  in  the  St.  Lawrence,  which  held  back  the  waters 
to  such  a  height  that  they  discharged  through  the  valley 
of  the  Mohawk  into  the  Hudson.  Furthermore,  at  some 
time  before  the  Glacial  period,  we  do  not  know  just  when, 
there  appears  to  have  been  an  old  Niagara  River,  now  filled 
with  drift,  which  ran  from  Lake  Erie  to  Ontario,  a  differ- 
ent channel  from  that  occupied  by  the  present  stream. 

The  effects  of  lakes  on  the  river  systems  with  which 
they  are  connected  is  in  many  ways  most  important.  Where 
they  are  of  considerable  extent,  or  where  even  small  they 
are  very  numerous,  they  serve  to  retain  the  flood  waters, 
delivering  them  slowly  to  the  excurrent  streams.  In  rising 
one  foot  a  lake  may  store  away  more  water  than  the  river 
by  its  consequent  rise  at  the  point  of  outflow  will  carry 
away  in  many  months,  and  this  for  the  simple  reason  that 
the  lake  may  be  many  hundred  or  even  thousand  times  as 
wide  as  the  stream.  Moreover,  as  before  noted,  the  sedi- 
ment gathered  by  the  stream  above  the  level  of  the  lake  is 
deposited  in  its  basin,  and  does  not  affect  the  lower  reaches 
of  the  river.  The  result  is  that  great  rivers,  such  as  drain 
from  the  Laurentian  Lakes,  flow  clear  water,  are  exempt 


206  OUTLINES  OF  THE  EARTH'S  HISTORY. 

from  floods,  are  essentially  without  alluvial  plains  or  ter- 
races, and  form  no  delta  deposits.  In  all  these  features  the 
St.  Lawrence  Kiver  ajffiords  a  wonderful  contrast  to  the 
Mississippi.  Moreover,  owing  to  the  clear  waters,  though 
it  has  flowed  for  a  long  time,  it  has  never  been  able  to  cut 
away  the  slight  obstructions  which  form  its  rapids,  bar- 
riers which  probably  would  have  been  removed  if  its  waters 
had  been  charged  with  sediment. 


"J^ 


CHAPTER  VI. 

GLACIEES. 

We  have  already  noted  the  fact  that  the  water  in  the 
clouds  is  very  commonly  in  the  frozen  state;  a  large  part 
of  that  fluid  which  is  evaporated  from  the  sea  attains  the 
solid  form  before  it  returns  to  the  earth.  Nevertheless, 
in  descending,  at  least  nine  tenths  of  the  precipitation 
returns  to  the  fluid  state,  and  does  the  kind  of  work  which 
we  have  noted  in  our  account  of  water.  Where,  however, 
the  water  arrives  on  the  earth  in  the  frozen  condition, 
it  enters  on  a  role  totally  different  from  that  followed  by 
the  fluid  material. 

Beginning  its  descent  to  the  earth  in  a  snowflake,  the 
little  mass  falls  slowly,  so  that  when  it  comes  against  the 
earth  the  blow  which  it  strikes  is  so  slight  that  it  does  no 
effective  work.  In  the  state  of  snow,  even  in  the  separate 
flakes,  the  frozen  water  contains  a  relatively  large  amount 
of  air.  It  is  this  air  indeed,  which,  by  dividing  the  ice  into 
many  flakes  that  reflect  the  light,  gives  it  the  white  colour. 
This  important  point  can  be  demonstrated  by  breaking 
transparent  ice  into  small  bits,  when  we  perceive  that  it  has 
the  hue  of  snow.  Much  the  same  effect  is  given  where  glass 
is  powdered,  and  for  the  same  reason. 

As  the  snowflakes  accumulate  layer  on  layer  they  imbed 
air  between  them,  so  that  when  the  material  falls  in  a 
feathery  shape — say  to  the  depth  of  a  foot — more  than  nine 
tenths  of  the  mass  is  taken  up  by  the  air-containing  spaces. 
As  these  cells  are  very  small,  the  circulation  in  them  is 
slight,  and  so  the  layer  becomes  an  admirable  non-con- 

207 


208  OUTLINES  OF  THE  EARTH'S  HISTORY. 

ductor,  having  this  quahty  for  the  same  reason  that  feath- 
ers have  it — i.  e.,  because  the  cells  are  small  enough  to 
prevent  the  circulation  of  the  air,  so  that  the  heat  which 
passes  has  to  go  by  conduction,  and  all  gases  are  very  poor 
conductors.  The  result  is  that  a  snow  coating  is  in  effect 
an  admirable  blanket.  When  the  sun  shines  upon  it,  much 
of  the  heat  is  reflected,  and  as  the  temperature  does  not 
penetrate  it  to  any  depth,  only  the  superficial  part  is 
melted.  This  molten  water  takes  up  in  the  process  of  melt- 
ing a  great  deal  of  heat,  so  that  when  it  trickles  down  into 
the  mass  it  readily  refreezes.  On  the  other  hand,  the  heat 
going  out  from  the  earth,  the  store  accumulated  in  its 
superficial  parts  in  the  last  warm  season,  together  with  the 
small  share  which  flows  out  from  the  earth's  interior,  is 
held  in  by  this  blanket,  which  it  melts  but  slowly.  Thus 
it  comes  about  that  in  regions  of  long-enduring  snowfall 
the  ground,  though  frozen  to  the  depth  of  a  foot  or  more 
at  the  time  when  the  accumulation  took  place,  may  be 
thawed  out  and  so  far  warmed  that  the  vegetation  begins 
to  grow  before  the  protecting  envelope  of  snow  has  melted 
away.  Certain  of  the  early  flowers  of  high  latitudes,  in- 
deed, begin  to  blossom  beneath  the  mantle  of  finely  di- 
vided ice. 

In  those  parts  of  the  earth  which  for  the  most  part 
receive  only  a  temporary  coating  of  snow  the  effect  of  this 
covering  is  inconsiderable.  The  snow  water  is  yielded  to 
the  earth,  from  which  it  has  helped  to  withdraw  the  frost, 
so  that  in  the  springtime,  the  growing  season  of  plants, 
the  ground  contains  an  ample  store  of  moisture  for  their 
development.  Where  the  snowfall  accumulates  to  a  great 
thickness,  especially  where  it  lodges  in  forests,  the  influence 
of  the  icy  covering  is  somewhat  to  protract  the  winter  and 
thus  to  abbreviate  the  growing  season. 

Where  snow  rests  upon  a  steep  slope,  and  gathers  to 
the  depth  of  several  feet,  it  begins  to  creep  slowly  down  the 
declivity  in  a  manner  which  we  may  often  note  on  house 
roofs.    This  motion  is  favoured  by  the  gradual  though  in- 


GLACIERS.  209 

complete  melting  of  the  flakes  as  the  heat  penetrates  the 
mass.  Making  a  section  through  a  mass  of  snow  which 
has  accumulated  in  many  successive  falls,  we  note  that  the 
top  may  still  have  the  flaky  character,  hut  that  as  we  go 
down  the  flakes  arc  replaced  by  adherent  shotlike  bodies, 
which  have  arisen  from  the  partial  melting  and' gathering 
to  their  centres  of  the  original  expanded  crystalline  bits. 
In  this  process  of  change  the  mass  can  move  particle  by 
particle  in  the  direction  in  which  gravity  impels  it.  The 
energy  of  its  motion,  however,  is  slight,  yet  it  can  urge  loose 
stones  and  forest  waste  down  hill.  Sometimes,  as  in  the 
cemetery  at  Augusta,  Me.,  where  stone  monuments  or  other 
structures,  such  as  iron  railings,  are  entangled  in  the  mov- 
ing mass,  it  may  break  them  off  and  convey  them  a  little 
distance  down  the  slope. 

So  long  as  the  summer  sun  melts  the  winter's  snow, 
even  if  the  ground  be  bare  but  for  a  day,  the  role  of  action 
accomplished  by  the  snowfall  is  of  little  geological  conse- 
quence. When  it  happens  that  a  portion  of  the  deposit 
holds  through  the  summer,  the  region  enters  on  the  glacial 
state,  and  its  conditions  undergo  a  great  revolution,  the 
consequences  of  which  are  so  momentous  that  we  shall 
have  to  trace  them  in  some  detail.  Fortunately,  the  con- 
siderations which  are  necessary  are  not  recondite,  and  all 
the  facts  are  of  an  extremely  picturesque  nature. 

Taking  such  a  region  as  New  England,  where  all  the 
earth  is  life-bearing  in  the  summer  season,  and  where  the 
glacial  period  of  the  winter  continues  but  for  a  short  time, 
we  find  that  here  and  there  on  the  high  mountains  the 
snow  endures  throughout  most  of  the  summer,  but  that 
all  parts  of  the  surface  have  a  season  when  life  springs 
into  activity.  On  the  top  of  Mount  Washington,  in  the 
White  Mountains  of  New  Hampshire,  in  a  cleft  known  as 
Tuckerman's  Ravine,  where  the  deposit  accumulates  to  a 
great  depth,  the  snow-ice  remains  until  midsummer.  It 
is,  indeed,  evident  that  a  very  slight  change  in  the  cli- 
matal  conditions  of  this  locality  would  establish  a  perma- 


210  OUTLIKES  OF  THE  EARTH'S  HISTORY. 

nent  accumulation  of  frozen  water  upon  the  summit  of 
the  mountain.  If  the  crest  were  lifted  a  thousand  feet 
higher,  without  any  general  change  in  the  heat  or  rainfall 
of  the  district,  this  effect  would  be  produced.  If  with  the 
same  amount  of  rainfall  as  now  comes  to  the  earth  in  that 
region  more  of  it  fell  as  snow,  a  like  condition  would  be 
established.  Furthermore,  with  an  increase  of  rainfall  to 
something  like  double  that  which  now  descends  the  snow 
bore  the  same  proportion  to  the  precipitation  which  it  does 
at  present,  we  should  almost  certainly  have  the  peak  above 
the  permanent  snow  line,  that  level  below  which  all  the 
winter's  fall  melts  away.  These  propositions  are  stated 
with  some  care,  for  the  reason  that  the  student  should  per- 
ceive how  delicate  may  be — indeed,  commonly  is — the  bal- 
ance of  forces  which  make  the  difference  between  a  seasonal 
and  a  perennial  snow  covering. 

As  soon  as  the  snow  outlasts  the  summer,  the  region 
which  it  occupies  is  sterilized  to  life.  From  the  time  the 
snow  begins  to  hold  over  the  warm  period  until  it  finally 
disappears,  that  field  has  to  be  reckoned  out  of  the  habitable 
earth,  not  only  to  man,  but  to  the  lowliest  organisms.* 

If  the  snow  in  a  glaciated  region  lay  where  it  fell,  the 
result  would  be  a  constant  elevation  of  the  deposit  year  by 
year  in  proportion  to  the  annual  excess  of  deposition  over 
the  melting  or  evaporation  of  the  material.  But  no  sooner 
does  the  deposit  attain  any  considerable  thickness  than  it 
begins  to  move  in  the  directions  of  least  resistance,  in  ac- 
cordance with  laws  which  the  students  of  glaciers  are  just 
beginning  to  discern.  In  small  part  this  motion  is  accom- 
plished by  avalanches  or  snow  slides,  phenomena  which 
are  in  a  way  important,  and  therefore  merit  description. 

*  In  certain  fields  of  permanent  snow,  particularly  near  their 
boundaries,  some  very  lowly  forms  of  vegetable  life  may  develop  on  a 
frozen  surface,  drawing  their  sustenance  from  the  air,  and  supplied 
with  water  by  the  melting  which  takes  place  during  the  summertime. 
These  forms  include  the  rare  phenomenon  termed  red  snow. 


GLACIERS.  211 

Immediately  after  a  heavy  snowfall,  in  regions  where  the 
slopes  are  steep,  it  often  happens  that  the  deposit  which 
at  first  clung  to  the  surface  on  which  it  lay  becomes  so 
heavy  that  it  tends  to  slide  down  the  slope;  a  trifling 
action,  the  slipping,  indeed,  of  a  single  flake,  may  begin 
the  movement,  Fhich  at  first  is  gradual  and  only  involves 
a  little  of  the  snow.  Gathering  velocity,  and  with  the 
materials  heaped  together  from  the  junction  of  that  already 
in  motion  with  that  about  to  be  moved,  the  avalanche  in 
sliding  a  few  hundred  feet  down  the  slope  may  become 
a  deep  stream  of  snow-ice,  moving  with  great  celerity. 
At  this  stage  it  begins  to  break  off  masses  of  ice  from  the 
glaciers  over  which  it  may  flow,  or  even  to  move  large 
stones.  Armed  with  these,  it  rends  the  underlying  earth. 
After  it  has  flowed  a  mile  it  may  have  taken  up  so  much 
earth  and  material  that  it  appears  like  a  river  of  mud. 
Owing  to  the  fact  that  the  energy  which  bears  it  downward 
is  through  friction  converted  into  heat,  a  partial  melting  of 
the  mass  may  take  place,  which  converts  it  into  what  we 
call  slush,  or  a  mixture  of  snow  and  water.  Finally,  the 
torrent  is  precipitated  into  the  bottom  of  a  valley,  where 
in  time  the  frozen  water  melts  away,  leaving  only  the  stony 
matter  which  it  bore  as  a  monument  to  show  the  termina- 
tion of  its  flow. 

It  was  the  good  fortune  of  the  writer  to  see  in  the  Swiss 
Oberland  one  very  great  avalanche,  which  came  from  the 
high  country  through  a  descent  of  several  thousand  feet  to 
the  surface  of  the  Upper  Grindelwald  Glacier.  The  first 
sign  of  the  action  was  a  vague  tremor  of  the  air,  like  that 
of  a  great  organ  pipe  when  it  begins  to  vibrate,  but  before 
the  pulsations  come  swiftly  enough  to  make  an  audible 
note.  It  was  impossible  to  tell  when  this  tremor  came,  but 
the  wary  guide,  noting  it  before  his  charge  could  perceive 
anything  unusual,  made  haste  for  the  middle  of  the  gla- 
cier. The  vibration  swelled  to  a  roar,  but  the  seat  of  the 
sound  amid  the  echoing  cliffs  was  indeterminable.  Finally, 
from  a  valley  high  up  on  the  southern  face  of  the  glacier, 


212  OUTLINES  OF  THE  EARTH'S  HISTORY. 

there  leaped  forth  first  a  great  stone,  which  sprang  with 
successive  rebounds  to  the  floor  of  ice.  Then  in  succession 
other  stones  and  masses  of  ice  which  had  outrun  the  flood 
came  thicker  and  thicker,  until  at  the  end  of  about  thirty 
seconds  the  steep  front  of  the  avalanche  appeared  like  a 
swift-moving  wall.  Attaining  the  cliffs,  it  shot  forth  as  a 
great  cataract,  which  during  the  continuance  of  the  flow — 
which  lasted  for  several  minutes — heaped  a  great  mound 
of  commingled  stones  and  ice  upon  the  surface  of  the  gla- 
cier. The  mass  thus  brought  down  the  steep  was  estimated 
at  about  three  thousand  cubic  yards,  of  which  probably 
the  fiftieth  part  was  rock  material.  An  avalanche  of  this 
volume  is  unusual,  and  the  proportion  of  stony  matter  borne 
down  exceptionally  great;  but  by  these  sudden  motions  of 
the  frozen  water  a  large  part  of  the  snow  deposited  above 
the  zone  of  complete  melting  is  taken  to  the  lower  valleys, 
where  it  may  disappear  in  the  summer  season,  and  much  of 
the  erosion  accomplished  in  the  mountains  is  brought  about 
by  these  falls. 

In  all  Alpine  regions  avalanches  are  among  the  most 
dreaded  accidents.  Their  occurrence,  however,  being  de- 
pendent upon  the  shape  of  the  surface,  it  is  generally  pos- 
sible to  determine  in  an  accurate  way  the  liability  of  their 
happening  in  any  particular  field.  The  Swiss  take  precau- 
tion to  protect  themselves  from  their  ravages  as  other  folk 
do  to  procure  immunity  from  floods.  Thus  the  authorities 
of  many  of  the  mountain  hamlets  maintain  extensive  for- 
ests on  the  sides  of  the  villages  whence  the  downfall  may  be 
expected,  experience  having  shown  that  there  is  no  other 
means  so  well  calculated  to  break  the  blow  which  these 
great  snowfalls  can  deliver,  as  thick-set  trees  which,  though 
they  are  broken  down  for  some  distance,  gradually  arrest 
the  stream. 

As  long  as  the  region  occupied  by  permanent  snow  is 
limited  to  sharp  mountain  peaks,  relief  by  the  precipitation 
of  large  masses  to  the  level  below  the  snow  line  is  easily 
accomplished,  but  manifestly  this  kind  of  a  discharge  can 


GLACIERS.  213 

only  be  effective  from  a  very  small  field.  Where  the  relief 
is  not  brought  about  by  these  tumbles  of  snow,  another 
mode  of  gravitative  action  accomplishes  the  result,  though 
in  a  more  roundabout  way,  through  the  mechanism  of 
glaciers. 

We  have  already  noted  the  fact  that  the  winter's  snow 
upon  our  hillsides  undergoes  a  movement  in  the  direction 
of  the  slope.  What  we  have  now  to  describe  in  a  rather 
long  story  concerning  glaciers  rests  upon  movements  of  the 
same  nature,  though  they  are  in  certain  features  peculiarly 
dependent  on  the  continuity  of  the  action  from  year  to 
year.  It  is  desirable,  however,  that  the  student  should  see 
that  there  is  at  the  foundation  no  more  mystery  in  glacial 
motion  than  there  is  in  the  gradual  descent  of  the  snow 
after  it  has  lain  a  week  on  a  hillside.  It  is  only  in  the  scale 
and  continuity  of  the  action  that  the  greatest  glacial  en- 
velope exceeds  those  of  our  temporary  winters — in  fact, 
whenever  the  snow  falls  the  earth  it  covers  enters  upon  an 
ice  period  which  differs  only  in  degree  from  that  from 
which  our  hemisphere  is  just  escaping. 

Where  the  reader  is  so  fortunate  as  to  be  able  to  visit 
a  region  of  glaciers,  he  had  best  begin  his  study  of  their 
majestic  phenomena  by  ascending  to  those  upper  realms 
where  the  snow  accumulates  from  year  to  year.  He  will 
there  find  the  natural  irregularities  of  the  rock  surface  in  a 
measure  evened  over  by  a  vast  sheet  of  snow,  from  which 
only  the  summits  of  the  greater  mountains  rise.  He  may 
soon  satisfy  himself  that  this  sheet  is  of  great  depth,  for 
here  and  there  it  is  intersected  by  profound  crevices.  If 
the  visit  is  made  in  the  season  when  snow  falls,  which  is 
commonly  during  most  of  the  year,  he  may  observe,  as 
before  noted  in  our  winter's  snow,  that  the  deposit,  though 
at  first  flaky,  attains  at  a  short  distance  below  the  surface 
a  somewhat  granular  character,  though  the  shotlike  grains 
fall  apart  when  disturbed.  Yet  deeper,  ordinarily  a  few 
feet  below  the  surface,  these  granules  are  more  or  less 
cemented  together;  the   mass  thus  loses   the   quality   of 


214  OUTLINES  OF  THE  EARTH'S  HISTORY. 

snow,  and  begins  to  appear  like  a  whitish  ice.  Looking 
down  one  of  the  crevices,  where  the  light  penetrates  to 
the  depth  of  a  hundred  feet  or  more,  he  may  see  that 
the  bluish  hue  somewhat  increases  with  the  depth.  A 
trace  of  this  colour  is  often  visible  even  in  the  surface 
snow  on  the  glacier,  and  sometimes  also  in  our  ordinary 
winter  fields.  In  a  hole  made  with  a  stick  a  foot  or  more 
in  depth  a  faint  cerulean  glimmer  may  generally  be  dis- 
cerned; but  the  increased  blueness  of  the  ice  as  we  go  down 
is  conspicuous,  and  readily  leads  us  to  the  conclusion  that 
the  air,  to  which,  as  we  before  noted,  the  whiteness  of  the 
snow  is  due,  is  working  out  of  the  mass  as  the  process  of 
compaction  goes  on.  In  a  glacial  district  this  snow  mass 
above  the  melting  line  is  called  the  neve. 

Eemembering  that  the  excess  of  snow  beyond  the  melt- 
ing in  a  neve  district  amounts,  it  may  be,  to  some  feet  of 
material  each  year,  we  easily  come  to  the  conclusion  that 
the  mass  works  down  the  slope  in  the  manner  which  it  does 
even  where  the  coating  is  impermanent.  This  supposition 
is  easily  confirmed:  by  observing  the  field  we  find  that  the 
sheet  is  everywhere  drawing  away  from  the  cliffs,  leaving 
a  deep  fissure  between  the  neve  and  the  precipices.  This 
crevice  is  called  by  the  German-Swiss  guides  the  Berg- 
schrund.  Passage  over  it  is  often  one  of  the  most  difficult 
feats  to  accomplish  which  the  Alpine  explorer  has  to 
undertake.  In  fact,  the  very  appearance  of  the  surface, 
which  is  that  of  a  river  with  continuous  down  slopes,  is 
sufficient  evidence  that  the  mass  is  slowly  flowing  toward 
the  valleys.  Following  it  down,  we  almost  always  come  to 
a  place  where  it  passes  from  the  upper  valleys  to  the  deeper 
gorges  which  pierce  the  skirts  of  the  mountain.  In  going 
over  this  projection  the  mass  of  snow-ice  breaks  to  pieces, 
forming  a  crowd  of  blocks  which  march  down  the  slope 
with  much  more  speed  than  they  journeyed  when  united  in 
the  higher-lying  fields.  In  this  condition  and  in  this  part 
of  the  movement  the  snow-ice  forms  what  are  called  the 
seracs,  or  curds,  as  the  word  means  in  the  French-Swiss 


GLACIERS.  215 

dialect.  Slipping  and  tumbling  down  the  steep  slope  on 
which  the  seracs  develop,  the  ice  becomes  broken  into  bits, 
often  of  small  size.  These  fragments  are  quickly  reknit 
into  the  body  of  ice,  which  we  shall  hereafter  term  the 
glacier,  and  in  this  process  the  expulsion  of  the  air  goes 
on  more  rapidly  than  before,  and  the  mass  assumes  a  more 
transparent  icelike^  quality. 

The  action  of  the  ice  in  the  pressures  and  strains  to 
which  it  is  subjected  in  joining  the  main  glacier  and  in  the 
further  part  of  its  course  demand  for  their  understanding  a 
revision  of  those  notions  as  to  rigidity  and  plasticity  which 
we  derive  from  our  common  experience  with  objects.  It 
is  hard  to  believe  that  ice  can  be  moulded  by  pressure  into 
any  shape  without  fracturing,  provided  the  motion  is 
slowly  effected,  while  at  the  same  time  it  is  as  brittle  as  ice 
to  a  sudden  blow.  We  see,  however,  a  similar  instance  of 
contrasted  properties  in  the  confection  known  as  molasses 
candy,  a  stick  of  which  may  be  indefinitely  bent  if  the  flex- 
ure is  slowly  made,  but  will  fly  to  pieces  like  glass  if  sharply 
struck.  Ice  differs  from  the  sugary  substance  in  many 
ways;  especially  we  should  note  that  while  it  may  be 
squeezed  into  any  form,  it  can  not  be  drawn  out,  but  frac- 
tures on  the  application  of  a  very  slight  tension.  The  con- 
ditions of  its  movement  we  will  inquire  into  further  on, 
when  we  have  seen  more  of  its  action. 

Entering  on  the  lower  part  of  its  course,  that  where  it 
flows  into  the  region  below  the  snow  line,  the  ice  stream 
is  now  confined  between  the  walls  of  the  valley,  a  channel 
which  in  most  cases  has  been  shaped  before  the  ice  time, 
by  a  mountain  torrent,  or  perhaps  by  a  slower  flowing  river. 
In  this  part  of  its  course  the  likeness  of  a  glacial  stream 
to  one  of  fluid  water  is  manifest.  We  see  that  it  twists 
with  the  turn  of  the  gorge,  widens  where  the  confining 
walls  are  far  apart,  and  narrows  where  the  space  is  con- 
stricted. Although  the  surface  is  here  and  there  broken 
by  fractures,  it  is  evident  that  the  movement  of  the  frozen 
current,  though  slow,  is  tolerably  free.  By  placing  stakes 
15 


210  OUTLINES  OF  THE  EARTH'S  HISTORY. 

in  a  row  across  the  axis  of  a  glacier,  and  observing  their 
movement  from  day  to  day,  or  even  from  hour  to  hour  if 
a  good  theodolite  is  used  for  the  purpose,  we  note  that  the 
movement  of  the  stream  is  fastest  in  the  middle  parts,  as  in 
the  case  of  a  river,  and  that  it  slows  toward  either  shore, 
though  it  often  happens,  as  in  a  stream  of  molten  water, 
that  the  speediest  part  of  the  current,  is  near  one  side. 
Further  observations  have  indicated  that  the  movement  is 
most  rapid  on  the  surface  and  least  at  the  bottom,  in  which 
the  stream  is  also  riverlike.  It  is  evident,  in  a  word,  that 
though  the  ice  is  not  fluid  in  strict  sense,  the  bits  of  which 
it  is  made  up  move  in.  substantially  the  manner  of  fluids — • 
that  is,  they  freely  slip  over  each  other.  We  will  now  turn 
our  attention  to  some  important  features  of  a  detailed  sort 
which  glaciers  exhibit. 

If  we  visit  a  glacier  during  the  part  of  the  year  when 
the  winter  snows  are  upon  it,  it  may  appear  to  have  a  very 
uninterrupted  surface.  But  as  the  summer  heat  advances, 
the  mask  of  the  winter  coating  goes  away,  and  we  may  then 
see  the  structure  of  the  ice.  First  of  all  we  note  in  all 
valley  glaciers  such  as  we  are  observing  that  the  stream 
is  overlaid  by  a  quantity  of  rocky  waste,  the  greater  part 
of  which  has  come  down  with  the  avalanches  in  the  man- 
ner before  described, -though  a  small  part  may  have  been 
worn  from  the  bed  over  which  the  ice  flows.  In  many 
glaciers,  particularly  as  we  approach  their  termination, 
this  sheet  of  earth  and  rock  materials  often  covers  the  ice 
so  completely  that  the  novice  in  such  regions  finds  it  diffi- 
cult to  believe  that  the  ice  is  under  his  feet.  If  the  ex- 
plorer is  minded  to  take  the  rough  scramble,  he  can  often 
walk  for  miles  on  these  masses  of  stone  without  seeing, 
much  less  setting  foot  on  any  frozen  water.  In  some  of 
the  Alaskan  glaciers  this  coating  may  bear  a  forest  growth. 
In  general,  this  material,  which  is  called  moraine,  is  dis- 
tributed in  bands  parallel  to  the  sides  of  the  glaciers,  and 
the  strips  may  amount  to  a  half  dozen  or  more.  Those  on 
the  sides  of  the  ice  have  evidently  been  derived  from  the 


GLACIERS. 


217 


precipices  which  they  have  passed.  Those  in  the  middle 
have  arisen  from  the  union  of  the  moraines  formed  in  two 
or  more  tributary  valleys. 

Where  the  avalanches  fall  most  plentifully,  the  stones 
lie  buried  with  the  snow,  and  only  melt  out  when  the 
stream  attains  the  region  where  the  annual  waste  of  its 
surface  exceeds  the  snowfall.  In  this  section  we  can  see 
how  the  progressive  melting  gradually  brings  the  rocky 
debris  into  plain  view.  Here  and  there  we  will  find  a 
boulder  perched  on  a  pedestal  of  ice,  which  indicates  a 


Fig.  12. — Map  of  glaciers  and  moraines  near  Mont  Blanc. 


recent  down-wearing  of  the  field.  A  frequent  sound  in 
these  regions  arises  from  the  tumble  of  the  stones  from 
their  pedestals  or  the  slipping  of  the  masses  from  the  sharp 
ridge  which  is  formed  by  the  protection  given  to  the  ice 
through  the  thick  coating  of  detritus  on  its  surface. 
These  movements  of  the  moraines  often  distribute  their 
waste  over  the  glacier,  so  that  in  its  lower  part  we  can  no 


218  OUTLINES  OF  THE  EARTH'S  HISTORY. 

longer  trace  the  contributions  from  the  several  valleys,  the 
whole  area  being  covered  by  the  debris.  At  the  end  of  the 
ice  stream,  where  its  forward  motion  is  finally  overcome 
by  the  warmth  which  it  encounters,  it  leaves  in  a  rude 
heap,  extending  often  like  a  wall  across  the  valley,  all  the 
coarse  fragments  which  it  conveys.  This  accumulation, 
composed  of  all  the  lateral  moraines  which  have  gathered 
on  the  ice  by  the  fall  of  avalanches,  is  called  the  terminal 
moraine.  As  the  ice  stream  itself  shrinks,  a  portion  of  the 
detritus  next  the  boundary  wall  is  apt  to  be  left  clinging 
against  those  slopes.  It  is  from  the  presence  of  these  heaps 
in  valleys  now  abandoned  by  glaciers  that  we  obtain  some 
information  as  to  the  former  greater  extent  of  glacial 
action. 

The  next  most  noticeable  feature  is  the  crevasse.  These 
fractures  often  exist  in  very  great  numbers,  and  constitute 
a  formidable  barrier  in  the  explorer's  way.  The  greater 
part  of  these  ruptures  below  the  serac  zone  run  from  the 
sides  of  the  stream  toward  the  centre  without  attaining 
that  region.  These  are  commonly  pointed  up  stream;  their 
formation  is  due  to  the  fact  that,  owing  to  the  swifter 
motion  in  the  central  parts  of  the  stream,  the  ice  in  that 
section  draws  away  from  the  material  which  is  moving 
more  slowly  next  the  shore.  As  before  noted,  these  ice 
fractures  when  drawn  out  naturally  form  fissures  at 
right  angles  to  the  direction  of  the  strain.  In  the  middle 
portions  of  the  ice  other  fissures  form,  though  more  rarely, 
which  appear  to  depend  on  local  strains  brought  about 
through  the  irregularity  of  the  surface  over  which  the  ice 
is  flowing. 

If  the  observer  is  fortunate,  he  may  in  his  journey  over 
the  glacier  have  a  chance  to  see  and  hear  what  goes  on 
when  crevasses  are  formed.  First  he  will  hear  a  deep, 
booming  sound  beneath  his  feet,  which  merges  into  a  more 
splintering  note  as  the  crevice,  which  begins  at  the  bottom 
or  in  the  distance,  comes  upward  or  toward  him.  When 
the  sound  is  over;  he  may  not  be  able  to  see  a  trace  of  the 


GLACIERS.  219 

fracture,  which  at  first  is  very  narrow.  But  if  the  break 
intersect  any  of  the  numerous  shallow  pools  which  in  a 
warm  summer's  day  are  apt  to  cover  a  large  part  of  the 
surface,  he  may  note  a  line  of  bubbles  rushing  up  through 
the  water,  marking  the  escape  of  the  air  from  the  glacier, 
some  remnant  of  that  which  is  imprisoned  in  the  original 
snow.  Even  where  this  indication  is  wanting,  he  can  some- 
times trace  the  crevice  by  the  hissing  sound  of  the  air 
streams  where  they  issue  from  the  ice.  If  he  will  take  time 
to  note  what  goes  on,  he  can  usually  in  an  hour  or  two  be- 
hold the  first  invisible  crack  widen  until  it  may  be  half  an 
inch  across.  He  may  see  how  the  surface  water  hastens 
down  the  opening,  a  little  river  system  being  developed  on 
the  surface  of  the  ice  as  the  streams  make  their  way  to  one 
or  more  points  of  descent.  In  doing  this  work  they  excavate 
a  shaft  which  often  becomes  many  feet  in  diameter,  down 
which  their  waters  thunder  to  the  base  of  the  glacier.  This 
well-like  opening  is  called  a  moulin,  or  mill,  a  name  which, 
as  we  shall  see,  is  well  deserved  from  the  work  which  fall- 
ing waters  accomplish.  Although  the  institution  of  the 
moulin  shaft  depends  upon  the  formation  of  a  crevice,  it 
often  happens  that  as  the  ice  moves  farther  on  its  journey 
its  walls  are  a^ain  thrust  together,  soldered  in  the  manner 
peculiar  to  ice,  so  that  no  trace  of  the  rupture  remains  ex- 
cept the  shaft  which  it  permitted  to  form.  Like  everything 
else  in  the  glacier,  the  moulin  slowly  moves  down  the  slope, 
and  remains  open  as  long  as  it  is  the  seat  of  descending 
waters  produced  by  the  summer  melting.  When  it  ceases 
to  be  kept  open  from  the  summer,  its  walls  are  squeezed 
together  in  the  fashion  that  the  crevices  are  closed. 

Forming  here  and  there,  and  generally  in  considerable 
numbers,  the  crevices  of  a  glacier  entrap  a  good  deal  of 
the  morainal  debris,  which  falls  through  them  to  the  bot- 
tom of  the  glacier.  Smaller  bits  are  washed  into  the  moulin 
by  the  streams  arising  from  the  melting  ice,  which  is 
brought  about  by  the  warm  sun  of  the  summer,  and  par- 
ticularly by  the  warm  rains  of  that  season.     On  those  gla- 


220  OUTLINES  OF  THE  EARTH'S  HISTORY. 

ciers  where,  owing  to  the  irregularity  of  the  bottom  over 
which  the  ice  flows,  these  fractures  are  very  numerous,  it 
may  happen  that  all  the  detritus  brought  upon  the  surface 
of  the  glacier  by  avalanches  finds  its  way  to  the  floor  of 
the  ice. 

Although  it  is  difficult  to  learn  what  is  going  on  at  the 
under  surface  of  the  glacier,  it  is  possible  directly  and  in- 
directly to  ascertain  much  concerning  the  peculiar  and  im- 
portant work  which  is  there  done.  The  intrepid  explorer  may 
work  his  way  in  through  the  lateral  fissures,  and  even  with 
care  safely  descend  some  of  the  fissures  which  penetrate  the 
central  parts  of  a  shallow  ice  stream.  There,  it  may  be  at 
the  depth  of  a  hundred  feet  or  more,  he  will  find  a  quantity 
of  stones,  some  of  which  may  be  in  size  like  to  a  small 
house  held  in  the  body  of  the  ice,  but  with  one  side  resting 
upon  the  bed  rock.  He  may  be  so  fortunate  as  to  see  the 
stone  actually  in  process  of  cutting  a  groove  in  the  bed 
rock  as  it  is  urged  forward  by  the  motion  of  the  glacier. 
The  cutting  is  not  altogether  in  the  fixed  material,  for  the 
boulder  itself  is  also  worn  and  scored  in  the  work.  Smaller 
pebbles  are  caught  in  the  space  between  the  erratic  and  the 
motionless  rock  and  ground  to  bits.  If  in  his  explorations 
the  student  finds  his  way  to  the  part  of  the  floor  on  which 
the  waters  of  a  moulin  fall,  he  may  have  a  chance  to  ob- 
serve how  the  stones  set  in  motion  serve  to  cut  the  bed  rock, 
forming  elongated  potholes  much  as  in  the  case  of  ordinary 
waterfalls,  or  at  the  base  of  those  shafts  which  afford  the 
beginnings  of  limestone  caverns. 

The  best  way  to  penetrate  beneath  the  glacier  is 
through  the  arch  of  the  stream  which  always  flows  from 
the  terminal  face  of  the  ice  river.  Even  in  winter  time 
every  large  glacier  discharges  at  its  end  a  considerable 
brook,  the  waters  of  which  have  been  melted  from  the  ice 
in  small  part  by  the  outflow  of  the  earth's  heat;  mainly, 
however,  by  the  warmth  produced  in  the  friction  of  the 
ice  on  itself  and  on  its  bottom — in  other  words,  by  the  con- 
version of  that  energy  of  position,  of  which  we  have  often 


GLACIERS.  221 

to  speak,  into  heat.  In  the  summer  time  this  subglacial 
stream  is  swollen  by  the  surface  waters  descending  through 
the  crevices  and  the  moulins  which  come  from  them,  so 
that  the  outflow  often  forms  a  considerable  river,  and  thus 
excavates  in  the  ice  a  large  or  at  least  a  long  cavern,  the  base 
of  which  is  the  bed  rock.  In  the  autumn,  when  the  super- 
ficial melting  ceases,  this  gallery  can  often  be  penetrated 
for  a  considerable  distance,  and  affords  an  excellent  way 
to  the  secrets  of  the  under  ice.  The  observer  may  here  see 
quantities  of  the  rock  material  held  in  the  grip  of  the  ice, 
and  forced  to  a  rude  journey  over  the  bare  foundation 
stones.  Now  and  then  he  may  find  the  glacial  mass  in  large 
measure  made  up  of  stones,  the  admixture  extending  many 
feet  above  the  bottom  of  the  cavern,  perhaps  to  the  very  top 
of  the  arch.  He  may  perchance  find  that  these  stones  are 
crushing  each  other  where  they  are  in  contact.  The  result 
will  be  brought  about  by  the  difference  in  the  rate  of  ad- 
vance of  the  ice,  which  moves  the  faster  the  higher  it  is 
above  the  surface  over  which  it  drags,  and  thus  forces  the 
stones  on  one  level  over  those  below.  Where  the  waters  of 
the  subglacial  stream  have  swept  the  bed  rock  clean  of 
debris  its  surface  is  scored,  grooved,  and  here  and  there 
polished  in  a  manner  which  is  accomplished  only  by  ice 
action,  though  some  likeness  to  it  is  afforded  where  stones 
have  been  swept  over  for  ages  by  blowing  sand.  Here  and 
there,  often  in  a  w^ay  which  interrupts  the  cavern  jour- 
ney, the  shrunken  stream,  unable  to  carry  forward  the 
debris,  deposits  the  material  in  the  chamber,  sometimes  fill- 
ing the  arch  so  completely  that  the  waters  are  forced  to 
make  a  detour.  This  action  is  particularly  interesting,  for 
the  reason  that  in  regions  whence  glaciers  have  disappeared 
the  deposits  formed  in  the  old  ice  arches  often  afford  sin- 
gularly perfect  moulds  of  those  caverns  which  were  pro- 
duced by  the  ancient  subglacial  streams.  These  moulds 
are  termed  esTcers. 

If  the  observer  be  attentive,  he  will  note  the  fact  that 
the  waters  emerging  from  beneath  the  considerable  glacier 


222  OUTLINES  OF  THE  EARTH'S  HISTORY. 

are  very  much  charged  with  mud.  If  he  will  take  a  glass 
of  the  water  at  the  point  of  escape,  he  will  often  find,  on 
permitting  it  to  settle,  that  the  sediment  amounts  to  as 
much  as  one  twentieth  of  the  volume.  While  the  greater 
part  of  this  detritus  will  descend  to  the  bottom  of  the  vessel 
in  the  course  of  a  day,  a  portion  of  it  does  not  thus  fall. 
He  may  also  note  that  this  mud  is  not  of  the  yellowish 
hue  which  he  is  accustomed  to  behold  in  the  materials 
laid  down  by  ordinary  rivers,  but  has  a  whitish  colour. 
Further  study  will  reveal  the  fact  that  the  difference  is 
due  to  the  lack  of  oxidation  in  the  case  of  the  glacial  de- 
tritus. Kiver  muds  forming  slowly  and  during  long-con- 
tinued exposure  to  the  action  of  the  air  have  their  con- 
tained iron  much  oxidized,  which  gives  them  a  part  of 
their  darkened  appearance.  Moreover,  they  are  somewhat 
coloured  with  decayed  vegetable  matter.  The  waste  from 
beneath  the  glacier  has  been  quickly  separated  from  the 
bed  rock,  all  the  faces  of  the  grains  are  freshly  fractured, 
and  there  is  no  admixture  of  organic  matter.  The  faces 
of  the  particles  thus  reflect  light  in  substantially  the  same 
way  as  powdered  glass  or  pulverized  ice,  and  consequently 
appear  white. 

A  little  observation  will  show  the  student  that  this  very 
muddy  character  of  waters  emerging  from  beneath  the 
glacier  is  essentially  peculiar  to  such  streams  as  we  have 
described.  Ascending  any  of  the  principal  valleys  of  Swit- 
zerland, he  may  note  that  some  of  the  streams  flow  waters 
which  carry  little  sediment  even  in  times  when  they  are 
much  swollen,  while  others  at  all  seasons  have  the  whitish 
colour.  A  little  further  exploration,  or  the  use  of  a  good 
map,  will  show  him  that  the  pellucid  streams  receive  no 
contributions  of  glacial  water,  while  those  which  look 
as  if  they  were  charged  with  milk  come,  in  part  at  least, 
from  the  ice  arches.  From  some  studies  which  the  writer 
has  made  in  Swiss  valleys,  it  appears  that  the  amount  of 
erosion  accomplished  on  equal  areas  of  similar  rock  by  the 
descent  of  the  waters  in  the  form  of  a  glacier  or  in  that 


GLACIERS.  223 

of  ordinary  torrents  differs  greatly.  Moving  in  the  form 
of  ice,  or  in  the  state  of  ice-confined  streams,  the  mass  of 
water  applies  very  many  times  as  much  of  its  energy  of 
position  to  grinding  and  bearing  away  the  rocks  as  is  ac- 
complished where  the  water  descends  in  its  fluid  state. 

The  effect  of  the  intense  ice  action  above  noted  is  rapid- 
ly to  wear  away  the  rocks  of  the  valley  in  which  the  glacier 
is  situated.  This  work  is  done  not  only  in  a  larger  measure 
but  in  a  different  way  from  that  accomplished  by  torrents. 
In  the  case  of  the  latter,  the  stream  bed  is  embarrassed 
by  the  rubbish  which  comes  into  it;  only  here  and  there 
can  it  attack  the  bed  rock  by  forcing  the  stones  over  its 
surface.  Only  in  a  few  days  of  heavy  rain  each  year  is  its 
work  at  all  effective;  the  greater  part  of  the  energy  of  posi- 
tion of  its  waters  is  expended  in  the  endless  twistings  and 
turnings  of  its  stream,  which  result  only  in  the  develop- 
ment of  heat  which  flies  away  into  the  atmosphere.  In  the 
ice  stream,  owing  to  its  slow  movement  and  to  the  detritus 
which  it  forces  along  the  bottom,  a  vastly  greater  part 
of  the  energy  which  impels  it  down  the  slope  is  applied 
to  rock  cutting.  None  of  the  boulders,  even  if  they  are 
yards  in  diameter,  obstruct  its  motion;  small  and  great 
alike  are  to  it  good  instruments  wherewith  to  attack  the 
bed  rocks.  The  fragments  are  never  left  to  waste  by  at- 
mospheric decay,  but  are  to  a  very  great  extent  used  up 
in  mechanical  work,  while  the  most  of  the  detritus  which 
comes  to  a  torrent  is  left  in  a  coarse  state  when  it  is  de- 
livered to  the  stream;  the  larger  part  of  that  which  the 
glacier  transports  is  worn  out  in  its  journey.  To  a  great 
extent  it  is  used  up  in  attacking  the  bed  rock.  In  most 
cases  the  debris  in  the  terminal  moraine  is  evidently  but  a 
small  part  of  what  entered  the  ice  during  its  journey  from 
the  uplands;  the  greater  part  has  been  worn  out  in  the 
rude  experiences  to  which  it  has  been  subjected. 

It  is  evident  that  even  in  the  regions  now  most  ex- 
tensively occupied  by  glaciers  the  drainage  systems  have 
been  shaped  by  the  movement  of  ordinary  streams — in  other 


224  OUTLINES  OF  THE  EARTH'S  HISTORY. 

words,  ice  action  is  almost  everywhere,  even  in  the  regions 
ahout  the  poles,  an  incidental  feature  in  the  work  of  water, 
coming  in  only  to  modify  tlie  topography,  which  is  mainly 
moulded  by  the  action  of  fluid  water.  When,  owing  to 
climatal  changes,  a  valley  such  as  those  of  the  Alps  is  oc- 
cupied by  a  glacial  stream,  the  new  current  proceeds  at 
once,  according  to  its  evident  needs,  to  modify  the  shape 
of  its  channel.  An  ordinary  torrent,  because  of  the  swift- 
ness of  its  motion,  which  may,  in  general,  be  estimated  at 
from  three  to  five  miles  an  hour,  can  convey  away  the  pre- 
cipitation over  a  very  narrow  bed.  Therefore  its  channel 
is  usually  not  a  hundredth  part  as  wide  as  the  gorge  or 
valley  in  which  it  lies.  But  when  the  discharge  takes 
place  by  a  glacier,  the  speed  of  which  rarely  exceeds  four 
or  five  feet  a  day,  the  ice  stream  because  of  its  slow  motion 
has  to  fdl  the  trough  from  side  to  side,  it  has  to  be  some 
thousand  times  as  deep  and  wide  as  the  torrent.  The  re- 
sult is  that  as  soon  as  the  glacial  condition  arises  in  a  coun- 
try the  ice  streams  proceed*  to  change  the  old  V-shaped 
torrent  beds  into  those  which  have  a  broad  U-like  form. 
The  practised  eye  can  in  a  way  judge  how  long  a  valley  has 
been  subjected  to  glacial  action  by  the  extent  to  which 
it  has  been  widened  by  this  process. 

In  the  valleys  of  Switzerland  and  other  mountain  dis- 
tricts which  have  been  attentively  studied  it  is  evident  that 
glacial  action  has  played  a  considerable  part  in  determin- 
ing their  forms.  But  the  work  has  been  limited  to  that 
part  of  the  basin  in  which  the  ice  is  abundantly  provided 
with  cutting  tools  in  the  stone  which  have  found  their 
way  to  the  base  of  the  stream.  In  the  region  of  the  neve, 
where  the  contributions  of  rocky  matter  to  the  surface  of 
the  deposit  made  from  the  few  bare  cliffs  which  rise  above 
the  sheet  of  snow  is  small,  the  snow-ice  does  no  cutting  of 
any  consequence.  Where  it  passes  over  the  steep  at  the 
head  of  the  deep  valley  into  which  it  drains,  and  is  riven 
into  the  seracs,  such  stony  matter  as  it  may  have  gath- 
ered is  allowed  to  fall  to  the  bottom,  and  so  comes  into  a 


GLACIERS.  225 

position  where  it  may  do  effective  work.  From  this  serac 
section  downward  the  now  distinct  ice  'river,  being  in  gen- 
eral below  the  snow  line,  has  everywhere  cliffs,  on  either 
side  from  which  the  contributions  of  rock  material  are 
abundant.  Hence  this  part  of  the  glacier,  though  it  is  the 
wasting  portion  of  its  length,  does  all  the  cutting  work  of 
any  consequence  which  is  performed.  It  is  there  that  the 
underrunning  streams  become  charged  with  sediment, 
which,  as  we  have  noted,  they  bear  in  surprising  quantities, 
and  it  is  therefore  in  this  section  of  the  valley  that  the  im- 
press of  the  ice  work  is  the  strongest.  Its  effect  is  not  only 
to  widen  the  valley  and  deepen  it,  but  also  to  advance  the 
deep  section  farther  up  the  stream  and  its  tributaries.  The 
step  in  the  stream  beds  which  we  find  at  the  scracs  appears 
to  mark  the  point  in  the  course  of  the  glacier  where,  owing 
to  the  falling  of  stones  to  its  base,  as  well  as  to  its  swifter 
movements  and  the  firmer  state  of  the  ice,  it  does  effective 
wearing. 

There  are  many  other  features  connected  with  glaciers 
which  richly  repay  the  study  of  those  who  have  a  mind 
to  explore  in  the  manner  of  the  physicist  interested  in  ice 
actions  the  difficult  problems  which  they  afford;  but  as 
these  matters  are  not  important  from  the  point  of  view  of 
this  work,  no  mention  of  them  will  here  be  made.  We  will 
now  turn  our  attention  to  that  other  group  of  glaciers  com- 
monly termed  continental,  which  now  exist  about  either 
pole,  and  which  at  various  times  in  the  earth's  history  have 
extended  far  toward  the  equator,  mantling  over  vast  ex- 
tents of  land  and  shallow  sea.  The  difference  between  the 
ice  streams  of  the  mountains  and  those  which  we  term  con- 
tinental depends  solely  on  the  areas  of  the  fields  and  the 
depth  of  the  accumulation.  In  an  ordinary  Alpine  region 
the  neve  districts,  where  the  snow  gathers,  are  relatively 
small.  Owing  to  the  rather  steep  slopes,  the  frozen  water 
is  rapidly  discharged  into  the  lower  valleys,  where  it  melts 
away.  Both  in  the  neve  and  in  the  distinct  glacier  of  the 
lower  grounds  there  are,  particularly  in  the  latter,  project- 


226  OUTLINES  OF  THE  EARTH'S  HISTORY. 

ing  peaks,  from  which  quantities  of  stone  are  brought  down 
by  avalanches  or  in  ordinary  rock  falls,  so  that  the  ice  is 
abundantly  supplied  with  cutting  tools,  which  work  from 
its  surface  down  to  its  depths. 

As  the  glacial  accumulation  grows  in  depth  there  are 
fewer  peaks  emerging  from  it,  and  the  streams  which  it 
feeds  rise  the  higher  until  they  mantle  over  the  divides 
between  the  valleys.  Thus  by  imperceptible  stages  valley 
glaciers  pass  to  the  larger  form,  usually  but  incorrectly 
termed  continental.  We  can,  indeed,  in  going  from  the 
mountains  in  the  tropics  to  the  poles,  note  every  step  in  this 
transition,  until  in  Greenland  we  attain  the  greatest  ice 
mass  in  the  w^orld,  unless  that  about  the  southern  pole  be 
more  extensive.  In  the  Greenland  glacier  the  ice  sheet 
covers  a  vast  extent  of  what  is  probably  a  mountain  coun- 
try, which  is  certainly  of  this  nature  in  the  southern  part 
of  the  island,  where  alone  we  find  portions  of  the  earth 
not  completely  covered  by  the  deep  envelope.  Thanks  to 
the  labours  of  certain  hardy  explorers,  among  whom  Nan- 
sen  deserves  the  foremost  place,  we  now  know  something 
as  to  the  conditions  of  this  vast  ice  field,  for  it  has  been 
crossed  from  shore  to  shore.  The  results  of  these  studies 
are  most  interesting,  for  they  afford  us  a  clew  as  to  the 
conditions  which  prevail  over  a  large  part  of  the  earth 
during  the  Glacial  period  from  which  the  planet  is  just 
escaping,  and  in  the  earlier  ages  when  glaciation  was  like- 
wise extensive.  We  shall  therefore  consider  in  a  somewhat 
detailed  way  the  features  which  the  Greenland  glacier  pre- 
sents. 

Starting  from  the  eastern  shore  of  that  land,  if  we  may 
thus  term  a  region  which  presents  itself  mainly  in  the  form 
of  ice,  we  find  next  the  shore  a  coast  line  not  completely 
covered  with  ice  and  snow,  but  here  and  there  exhibiting 
peaks  which  indicate  that  if  the  frozen  m-antle  were  re- 
moved the  country  would  appear  deeply  intersected  w4th 
fiords  in  the  manner  exhibited  in  the  regions  to  the  south 
of  Greenland  or  the   Scandinavian  peninsula.     The  ice 


GLACIERS.  227 

comes  down  to  the  sea  through  the  valleys,  often  facing 
the  ocean  for  great  distances  with  its  frozen  cliffs.  En- 
tering on  this  seaward  portion  of  the  glacier,  the  ob- 
server finds  that  for  some  distance  from  the  coast  line  the 
ice  is  more  or  less  rifted  with  crevices,  the  formation  of 
which  is  doubtless  due  to  irregularities  of  the  rock  bottom 
over  which  it  moves.  These  ruptures  are  so  frequent  that 
for  some  miles  back  it  is  very  difficult  to  find  a  safe  way. 
Finally,  however,  a  point  is  attained  where  these  breaks 
rather  suddenly  disappear,  and  thence  inward  the  ice  rises 
at  the  rate  of  upward  slope  of  a  few  feet  to  the  mile  in  a 
broad,  nearly  smooth  incline.  In  the  central  portion  of 
the  region  for  a  considerable  part  of  the  territory  the  ice 
has  very  little  slope.  Thence 'it  declines  toward  the  other 
shore,  exhibiting  the  same  features  as  were  found  on  the 
eastern  versant  until  near  the  coast,  when  again  the  sur- 
face is  beset  with  crevices  which  continue  to  the  margin 
of  the  sea. 

Although  the  explorations  of  the  central  field  of  Green- 
land are  as  yet  incomplete,  several  of  these  excursions  into 
or  across  the  interior  have  been  made,  and  the  identity 
of  the  observations  is  such  that  we  can  safely  assume  the 
whole  region  to  be  of  one  type.  We  can  furthermore  run 
no  risk  in  assuming  that  what  we  find  in  Greenland,  at 
least  so  far  as  the  unbroken  nature  of  the  central  ice  field 
is  concerned,  is  what  must  exist  in  Qvery  land  where  the 
glacial  envelope  becomes  very  deep.  In  Greenland  it  seems 
likely  that  the  depth  of  the  ice  is  on  the  average  more 
than  half  a  mile,  and  in  the  central  part  of  the  realm  the 
sheet  may  well  have  a  much  greater  profundity;  it  may  be 
nearly  a  mile  deep.  The  most  striking  feature — that  of  a 
vast  unbroken  expanse,  bordered  by  a  region  where  the  ice 
is  ruptured — is  traceable  wherever  very  extensive  and  pre- 
sumably deep  deposits  of  ice  have  been  examined.  As 
we  shall  see  hereafter,  these  features  teach  us  much  as  to 
the  conditions  of  glacial  action — a  matter  which  we  shall 
have  to  examine  after  we  have  completed  our  general 


228  OUTLINES  OF  THE  EARTH'S  HISTORY. 

survey   as   to    the    changes   which    occur   during   glacial 
periods. 

In  the  present  state  of  that  wonderful  complex  of. 
actions  which  we  term  climate,  glaciers  are  everywhere,  so 
far  as  our  observations  enable  us  to  judge,  generally  in  pro- 
cess of  decrease.  In  Switzerland,  although  the  ancients 
even  in  Eoman  days  were  in  contact  with  the  ice,  they  were 
so  unobservant  that  they  did  not  even  remark  that  the  ice 
was  in  motion.  Only  during  the  last  two  centuries  have  we 
any  observations  of  a  historic  sort  which  are  of  value  to 
the  geologist.  Fortunately,  however,  the  signs  written  on 
the  rock  tell  the  story,  except  for  its  measurement  in  terms 
of  years,  as  clearly  as  any  records  could  give  it.  From  this 
testimony  of  the  rocks  we  perceive  that  in  the  geological 
yesterday,  though  it  may  have  been  some  tens  of  thousands 
of  years  ago,  the  Swiss  glaciers,  vastly  thickened,  and  with 
their  horizontal  area  immensely  expanded,  stretched  over 
the  Alpine  countr}^  so  that  only  here  and  there  did  any 
of  the  sharper  peaks  rise  above  the  surface.  These  vast 
glaciers,  almost  continually  united  on  their  margins,  ex- 
tended so  far  that  every  portion  of  what  is  now  the  Swiss 
Republic  was  covered  by  them.  Their  front  lay  on  the 
southern  lowlands  of  Germany,  on  the  Jura  district  of 
France;  on  the  sovith,  it  stretched  across  the  valley  of  the 
Po  as  far  as  near  Milan.  We  know  this  old  ice  front  by  the 
.accumulations  of  rock  debris  which  were  brought  to  it  from 
the  interior  of  the  mountain  realm.  We  can  recognise  the 
peculiar  kinds  of  stone,  and  with  perfect  certainty  trace 
them  to  the  bed  rock  whence  they  were  riven.  Moreover, 
we  can  follow  back  through  the  same  evidence  the  stages 
of  retreat  of  the  glaciers,  until  they  lost  their  broad  conti- 
nental character  and  assumed  something  like  their  present 
valley  form.  Up  the  valley  of  any  of  the  great  rivers,  as, 
for  instance,  that  of  the  Rhone  above  the  lake  of  Geneva, 
we  note  successive  terminal  moraines  which  clearly  indi- 
cate stages  in  the  retreat  of  the  ice  when  for  a  time  it 
ceased  to  go  backward,  or  even  made  a  slight  temporary 


GLACIERS.  229 

readvance.  It  is  easily  seen  that  on  such  occasions  the 
stones  carried  to  the  ice  front  would  be  accumulated  in  a 
heap,  while  during  the  time  when  day  by  day  the  glacier 
was  retreating  the  rock  waste  would  be  left  broadcast  over 
the  valley. 

As  we  go  up  from  the  course  of  the  glacial  streams  we 
note  that  the  successive  moraines  have  their  materials  in 
a  progressively  less  decayed  state.  Far  away  from  the  heap 
now  forming,  and  in  proportion  to  the  distance,  the  stones 
have  in  a  measure  rotted,  and  the  heaps  which  they  com- 
pose are  often  covered  with  soil  and  occupied  by  forests. 
Within  a  few  miles  of  the  ice  front  the  stones  still  have 
a  fresh  aspect.  When  we  arrive  within,  say,  half  a  mile 
of  the  moraine  now  building,  we  come  to  the  part  of  the 
glacial  retreat  of  which  we  have  some  written  or  tradi- 
tional account.  This  is  in  general  to  the  effect  that  the 
wasting  of  the  glaciers  is  going  on  in  this  century  as  it 
went  on  in  the  past.  Occasionally  periods  of  heavy  snow 
would  refresh  the  ice  streams,  so  that  for  a  little  time  they 
pushed  their  fronts  farther  down  the  valley.  The  writer 
has  seen  during  one  of  these  temporary  advances  the  inter- 
esting spectacle  of  ice  destroying  and  overturning  the  soil 
of  a  small  field  which  had  been  planted  in  grain. 

It  should  be  noted  that  these  temporary  advances  of  the 
ice  are  not  due  to  the  snowfall  of  the  winter  or  winters  im- 
mediately preceding  the  forward  movement.  So  slow  is  the 
journey  of  the  ice  from  the  neve  field  to  the  end  of  a  long 
glacier  that  it  may  require  centuries  for  the  store  accumu- 
lated in  the  uplands  to  affect  the  terminal  portion  of  the 
stream.  We  know  that  the  bodies  of  the  unhappy  men 
who  have  been  lost  in  the  crevices  of  the  glacier  are  borne 
forward  at  a  uniform  and  tolerably  computable  rate  until 
they  emerge  at  the  front,  where  the  ice  melts  away.  In 
at  least  one  case  the  remains  have  appeared  after  many 
years  in  the  debris  which  is  contributed  to  the  moraine.  On 
account  of  this  slow  feeding  of  the  glacial  stream,  we  natu- 
rally may  expect  to  find,  as  we  do,  in  fact,  that  a  great 


230  OUTLINES  OF  THE  EARTH'S  HISTORY. 

snowfall  of  many  years  ago,  and  likewise  a  period  when 
the  winter's  contribution  has  been  slight,  would  influence 
the  position  of  the  terminal  point  of  the  ice  stream  at 
different  times,  according  to  its  length.  If  the  length  of 
the  flow  be  five  miles,  it  may  require  twenty  or  thirty  years 
for  the  effect  to  be  evident;  while  if  the  stream  be  ten  miles 
long,  the  influence  may  not  be  noted  in  less  than  three- 
score years.  Thus  it  comes  about  that  at  the  present  time 
in  the  same  glacial  district  some  streams  may  be  advancing 
while  others  are  receding,  though,  on  the  whole,  the  ice 
is  generally  in  process  of  shrinkage.  If  the  present  rate  of 
retreat  should  be  maintained,  it  seems  certain  that  at  the 
end  of  three  centuries  the  Swiss  glaciers  as  a  whole  will 
not  have  anything  like  their  present  area,  and  many  of  the 
smaller  streams  will  entirely  disappear. 

Following  the  method  of  the  illustrious  Louis  Agassiz, 
who  first  attentively  traced  the  evidence  which  shows  the 
geologically  recent  great  extension  of  glaciers  by  studying 
the  evidence  of  the  action  in  fields  they  no  longer  occupy, 
geologists  have  now  inspected  a  large  part  of  the  land  areas 
with  a  view  to  finding  the  proofs  of  such  ice  work.  So 
far  as  these  indications  are  concerned,  the  indications  which 
they  have  had  to  trace  are  generally  of  a  very  unmistakable 
character.  Rarely,  indeed,  does  a  skilled  student  of  such 
phenomena  have  to  search  in  any  region  for  more  than  a 
day  before  he  obtains  indubitable  evidence  which  will  en- 
able him  to  determine  whether  or  not  the  field  has  recently 
been  occupied  by  an  enduring  ice  sheet — one  which  survives 
the  summer  season  and  therefore  deserves  the  name  of 
glacier.  The  indications  which  he  has  to  consider  consist 
in  the  direction  and  manner  in  which  the  surface  mate- 
rials have  been  carried,  the  physical  conditions  of  these 
materials,  the  shape  of  the  surface  of  the  underlying  rock 
as  regards  its  general  contour,  and  the  presence  or  absence 
of  scratches  and  groovings  on  its  surface.  As  these  records 
of  ice  action  are  of  first  importance  in  dealing  with  this 
problem,  and  as  they  afford  excellent  subjects  for  the  study 


GLACIERS.  231 

of  those  who  dwell  in  glaciated  regions,  we  shall  note  them 
in  some  detail. 

The  geologist  recognises  several  ways  in  which  mate- 
rials may  be  transported  on  the  surface  of  the  earth.  They 
may  be  cast  forth  by  volcanoes,  making  their  journey  by 
being  shot  through  the  air,  or  by  flowing  in  lava  streams; 
it  is  always  easy  at  a  glance,  save  in  very  rare  instances, 
to  determine  whether  fragments  have  thus  been  conveyed. 
Again,  the  detritus  may  be  moved  by  the  wind;  this  action 
is  limited;  it  only  affects  dust,  sand,  and  very  small  peb- 
bles, and  is  easily  discriminated.  The  carriage  may  be 
effected  by  river  or  marine  currents;  here,  again,  the  size 
of  the  fragments  moved  is  small,  and  the  order  of  their 
arrangement  distinctly  traceable.  The  fragments  may  be 
conveyed  by  ice  rafts;  here,  too,  the  obs'erver  can  usually 
limit  the  probabilities  he  has  to  consider  by  ascertaining, 
as  he  can  generally  do,  whether  the  region  which  he  is 
observing  has  been  below  a  sea  or  lake.  In  a  word,  the 
before-mentioned  agents  of  transportation  are  of  somewhat 
exceptional  influence,  and  in  most  cases  can,  as  explanations 
of  rock  transportation,  be  readily  excluded.  When,  there- 
fore, the  geologist  finds  a  country  abundantly  covered  with 
sand,  pebbles,  and  boulders  arranged  in  an  irregular  way, 
he  has  generally  only  to  inquire  whether  the  material  has 
been  carried  by  rivers  or  by  glaciers.  This  discrimination 
can  be  quickly  and  critically  effected.  In  the  first  place, 
he  notes  that  rivers  only  in  their  torrent  sections  can  carry 
large  fragments  of  rock,  and  that  in  all  cases  the  fragments 
move  down  hill.  Further,  that  where  deposits  are  formed, 
they  have  more  or  less  the  form  of  alluvial  deposits.  If 
now  the  observations  show  that  the  rock  waste  occupying 
the  surface  of  any  region  has  been  carried  up  hill  and 
down,  across  the  valleys,  particularly  if  there  are  here  and 
there  traces  of  frontal  moraines,  the  geologist  is  entitled 
to  suppose — he  may,  indeed,  be  sure — that  the  carriage 
has  been  effected  by  a  glacial  sheet. 

Important  corroborative  evidence  of-  ice  action  is  gen- 
16 


232         OUTLINES  OF  THE  EARTH'S  HISTORY. 

erally  to  be  found  by  inspecting  the  bed  rock  below  the 
detritus,  which  indicates  glacial  action.  Even  if  it  be 
somewhat  decayed,  as  is  apt  to  be  the  case  where  the  ice 
sheet  long  since  passed  away,  the  bed  rock  is  likely  to  have 
a  warped  surface;  it  is  cast  into  ridges  and  furrows  of  a 
broad,  flowing  aspect,  such  as  liquid  water  never  produces, 
which,  indeed,  can  only  be  created  by  an  ice  sheet  moving 
over  the  surface,  cutting  its  bed  in  proportion  to  the  hard- 
ness of  the  material.  Furthermore,  if  the  bed  rock  have 
a  firm  texture,  and  be  not  too  much  decayed,  we  almost 
always  find  upon  it  grooves  or  scratches,  channels  carved 
by  the  stones  embedded  in  the  body  of  the  ice,  and  drawn 
by  its  motion  over  the  fixed  material.  Thus  the  proof  of 
glacial  extension  in  the  last  ice  epoch  is  made  so  clear  that 
accurate  maps  can  be  prepared  showing  the  realm  of  its 
action.  This  task  is  as  yet  incomplete,  although  it  is  al- 
ready far  advanced. 

While  the  study  of  glaciers  began  in  Europe,  inquiries 
concerning  their  ancient  extension  have  been  carried  fur- 
ther and  with  more  accuracy  in  North  America  than  in 
any  other  part  of  the  world.  We  may  therefore  well 
begin  our  description  of  the  limits  of  the  ice  sheets  with 
this  continent.  Imagining  a  seafarer  to  have  approached 
America  by  the  North  Atlantic,  as  did  the  Scandinavians, 
and  that  his  voyage  came  perhaps  a  hundred  thousand 
years  or  more  before  that  of  Leif  Ericsson,  he  would 
have  found  an  ice  front  long  before  he  attained  the  pres- 
ent shores  of  the  land.  This  front  may  have  extended 
from  south  of  Greenland,  off  the  shores  of  the  present 
Grand  Banks  of  Newfoundland,  thence  and  westward  to 
central  or  southern  New  Jersey.  This  cliff  of  ice  was 
formed  by  a  sheet  which  lay  on  the  bottom  of  the  seai 
On  the  New  Jersey  coast  the  ice  wall  left  the  sea  and  en- 
tered on  the  body  of  the  continent.  We  will  now  suppose 
that  the  explorer,  animated  with  the  valiant  scientific 
spirit  which  leads  the  men  of  our  day  to  seek  the  poles, 
undertook  a  land  journey  along  the  ice  front  across  the 


GLACIERS.  233 

continent.  From  the  New  Jersey  coast  the  traveller  would 
have  passed  through  central  Pennsylvania^  where,  although 
there  probably  detached  outlying  glaciers  lying  to  the 
southward  as  far  as  central  Virginia,  the  main  front  ex- 
tended westward  into  the  Ohio  Valley.  In  southern  Ohio 
a  tongue  of  the  ice  projected  southwardly  until  it  crossed 
the  Ohio  Eiver,  where  Cincinnati  now  lies,  extending  a  few 
miles  to  the  southward  of  the  stream.  Thence  it  deflected 
northwardly,  crossing  the  Mississippi,  and  again  the  Mis- 
souri, with  a  tongue  or  lobe  which  went  far  southward  in 
that  State.  Then  again  turning  to  the  northwest,  it  fol- 
lowed in  general  the  northern  part  of  the  Missouri  basin 
until  it  came  to  within  sight  of  the  Rocky  Mountains. 
There  the  ice  front  of  the  main  glacier  followed  the  trend 
of  the  mountains  at  some  distance  from  their  face  for  an 
unknown  extent  to  the  northward.  In  the  Cordilleras,  as 
far  south  as  southern  Colorado,  and  probably  in  the  Sierra 
Nevada  to  south  of  San  Francisco,  the  mountain  centres 
developed  local  glaciers,  which  in  some  places  were  of  very 
great  size,  perhaps  exceeding  any  of  those  which  now  exist 
in  Switzerland.  It  will  thus  be  seen  that  nearly  one  half 
of  the  present  land  area  of  North  America  was  beneath  a 
glacial  covering,  though,  as  before  noted,  the  region  about 
the  Gulf  of  Mexico  may  have  swayed  upward  when  the 
northern  portion  of  the  land  was  borne  down  by  the  vast 
load  of  ice  which  rested  upon  it.  Notwithstanding  this 
possible  addition  to  the  land,  our  imaginary  explorer  would 
have  found  the  portion  of  the  continent  fit  for  the  occu- 
pancy of  life  not  more  than  half  as  great  as  it  is  at  present. 
In  the  Eurasian  continent  there  was  no  such  continuous 
ice  sheet  as  in  North  America,  but  the  glaciers  developed 
from  a  number  of  different  centres,  each  moving  out  upon 
the  lowlands,  or,  if  its  position  was  southern,  being  limited 
to  a  particular  mountain  field.  One  of  these  centres  in- 
cluded Scandinavia,  northern  Germany,  Great  Britain 
about  as  far  south  as  London,  and  a  large  part  of  Ireland, 
the  ice  covering  the  intermediate  seas  and  extending  to 


234  OUTLINES  OF  THE  EARTH'S  HISTORY. 

the  westward,  so  that  the  passage  of  the  North  Atlantic 
was  greatly  restricted  between  this  ice  front  and  that  of 
North  America.  Another  centre,  before  noted,  was  formed 
in  the  Alps;  yet  another,  of  considerable  area,  in  the 
Pyrenees;  other  less  studied  fields  existed  in  the  Apen- 
nines, in  the  Caucasus,  the  Ural,  and  the  other  moun- 
tains of  northern  Asia.  Curiously  enough,  however,  the 
great  region  of  plains  in  Siberia  does  not  appear  to  have 
been  occupied  by  a  continuous  ice  sheet,  though  the  simi- 
lar region  in  North  America  was  deeply  embedded  in  a 
glacier.  Coincident  with  this  development  of  ice  in  the 
eastern  part  of  the  continent,  the  ice  streams  of  the  Hima- 
layan Mountains,  some  of  which  are  among  the  greatest 
of  our  upland  glaciers,  appear  to  have  undergone  but  a 
moderate  extension.  Many  other  of  the  Eurasian  high- 
lands were  probably  ice-bound  during  the  last  Glacial 
period,  but  our  knowledge  concerning  these  local  fields 
is  as  yet  imperfect. 

In  the  southern  hemisphere  the  lands  are  of  less  extent 
and,  on  the  whole,  less  studied  than  in  the  northern  realm. 
Here  and  there  where  glaciers  exist,  as  in  New  Zealand 
and  in  the  southern  part  of  South  America,  observant  trav- 
ellers have  noticed  that  these  ice  fields  have  recently  shrunk 
away.  Whether  the  time  of  greatest  extension  and  of  re- 
treat coincided  with  that  of  the  ice  sheets  in  the  north 
is  not  yet  determined;  the  problem,  indeed,  is  one  of  some 
difficulty,  and  may  long  remain  undecided.  It  seems, 
however,  probable  that  the  glaciers  of  the  southern  hemi- 
sphere, like  those  in  the  north,  are  in  process  of  retreat. 
If  this  be  true,  then  their  time  of  greatest  extension  was 
probably  the  same  as  that  of  the  ice  sheets  about  the  south- 
ern pole.  From  certain  imperfect  reports  which  we  have 
concerning  evidences  of  glaciation  in  Central  America  and 
in  the  Andean  district  in  the  northern  part  of  South 
America,  it  seems  possible  that  at  one  time  the  upland  ice 
along  the  Cordilleran  chain  existed  from  point  to  point 
along  that  system  of  elevations,  so  that  the  widest  interval 


GLACIERS.  235 

between  the  fields  of  permanent  snow  with  their  attendant 
glaciers  did  not  much  exceed  a  thousand  miles. 

Observing  the  present  gradual  retreat  of  those  ice  rem- 
nants which  remain  mere  shreds  and  patches  of  the  an- 
cient fields,  it  seems  at  first  sight  likely  that  the  extension 
and  recession  of  the  great  glaciers  took  place  with  exceed- 
ing slowness.  Measured  in  terms  of  human  life,  in  the 
manner  in  which  we  gauge  matters  of  man's  history,  this 
process  was  doubtless  slow.  There  are  reasons,  however, 
to  believe  that  the  coming  and  going  were,  in  a  geological 
sense,  swift;  they  may  have,  indeed,  been  for  a  part  of  the 
time  of  startling  rapidity.  Going  back  to  the  time  of  geo- 
logical yesterday,  before  the  ice  began  its  development  in 
the  northern  hemisphere,  all  the  evidence  we  can  find  ap- 
pears to  indicate  a  temperate  climate  extending  far  toward 
the  north  pole.  The  Miocene  deposits  found  within  twelve 
degrees,  or  a  little  more  than  seven  hundred  miles,  of  the 
north  pole,  and  fairly  within  the  realm  of  lowest  tempera- 
ture which  now  exists  on  the  earth,  show  by  the  plant  re- 
mains which  they  contain  that  the  conditions  permitted 
the  growth  of  forests,  the  plants  having  a  tolerably  close 
resemblance  to  those  which  now  freely  develop  in  the 
southern  portion  of  the  Mississippi  Valley.  Among  them 
there  are  species  which  had  the  habit  of  retaining  their 
broad,  rather  soft  leaves  throughout  the  winter  season. 
The  climate  appears,  in  a  word,  to  have  been  one  where 
the  mean  annual  temperature  must  have  been  thirty  de- 
grees or  more  higher  than  the  present  average  of  that 
realm.  Although  such  conditions  near  the  sea  level  are 
not  inconsistent  with  the  supposition  that  glaciers  existed 
in  the  higher  mountains  of  the  north,  they  clearly  deny 
the  possibility  of  the  realm  being  occupied  by  continental 
glaciers. 

Although  the  Pliocene  deposits  formed  in  high  latitudes 
have  to  a  great  extent  been  swept  away  by  the  subsequent 
glacial  wearing,  they  indicate  by  their  fossils  a  climatal 
change  in  the  direction  of  greater  cold.     We  trace  this 


236  OUTLINES  OF  THE  EARTH'S  HISTORY. 

change,  though  obscurely,  in  a  progressive  manner  to  a 
point  where  the  records  are  interrupted,  and  the  next  inter- 
pretable  indication  we  have  is  that  the  ice  sheet  had  ex- 
tended to  somewhere  near  the  Hmits  which  we  have  noted. 
We  are  then  driven  to  seek  what  we  can  concerning  the 
sojourn  of  the  ice  on  the  land  by  the  amount  of  wearing 
which  it  has  inflicted  upon  the  areas  w^hich  it  occupied. 
This  evidence  has  a  certain,  though,  as  we  shall  see,  a  lim- 
ited value. 

When  the  students  of  glacial  action  first  began  the  great 
task  of  interpreting  these  records,  they  were  led  to  suppose 
that  the  amount  of  rock  cutting  which  was  done  by  the  ice 
was  very  great.  Observing  what  goes  on,  in  the  manner 
we  have  noted,  beneath  a  valley  glacier  such  as  those  of 
Switzerland,  they  saw  that  the  ice  work  went  on  rapidly, 
and  concluded  that  if  the  ice  remained  long  at  work  in  a 
region  it  must  do  a  vast  deal  of  erosion.  They  were  right 
in  a  part  of  their  premises,  but,  as  we  shall  see,  probably 
in  another  part  wrong.  Looking  carefully  over  the  field 
where  the  ice  has  operated,  w^e  note  that,  though  at  first 
sight  the  area  appears  to  have  lost  all  trace  of  its  pre- 
glacial  river  topograph}^  this  aspect  is  due  mainly  to  the 
irregular  way  in  which  the  glacial  waste  is  laid  down.  Close 
study  shows  us  that  we  may  generally  trace  the  old  stream 
valleys  down  to  those  which  were  no  larger  than  brooks. 
It  is  true  that  these  channels  are  generally  and  in  many 
places  almost  altogether  filled  in  with  rubbish,  but  a  close 
study  of  the  question  has  convinced  the  writer,  and  this 
against  a  previous  view,  that  the  amount  of  erosion  in  New 
England  and  Canada,  where  the  work  was  probably  as 
great  as  anywhere,  has  not  on  tte  average  exceeded  a  hun- 
dred feet,  and  probably  was  much  less  than  that  amount. 

Even  in  the  region  north  of  Lake  Ontario,  over  which 
the  ice  was  deep  and  remained  for  a  long  time,  the  amount 
of  erosion  is  singularly  small.  Thus  north  of  Kingston  the 
little  valleys  in  the  limestone  rocks  which  were  cut  by  the 
preglacial    streams,    though    somewhat    encumbered   with 


GLACIERS.  237 

drift,  remain  almost  as  distinct  as  they  are  on  similar  strata 
in  central  Kentucky,  well  south  of  the  field  which  the  ice 
occupied.  In  fact,  the  ice  sheet  appears  to  have  done  the 
greatest  part  of  its  work  and  to  have  affected  the  surface 
most  in  the  belt  of  country  a  few  hundred  miles  in  width 
around  the  edges  of  the  sheet.  It  was  to  be  expected  that 
in  a  continental  glacier,  as  in  those  of  mountain  valleys, 
the  most  of  the  debris  should  be  accumulated  about  the 
margin  where  the  materials  dropped  from  the  ice.  But 
why  the  cutting  action  should  be  greatest  in  that  marginal 
field  is  not  at  first  sight  clear.  To  explain  this  and  other 
features  as  best  we  may,  we  shall  now  consider  the  prob- 
able history  of  the  great  ice  march  in  advance  and  retreat, 
and  then  take  up  the  conditions  which  brought  about  its 
develo{)ment  and  its  disappearance. 

Ice  is  in  many  ways  the  most  remarkable  substance 
with  which  the  physicist  has  to  deal,  and  among  its  emi- 
nent peculiarities  is  that  it  expands  in  freezing,  while  the 
rule  is  that  substances  contract  in  passing  from  the  fluid 
to  the  solid  state.  On  this  account  frozen  water  acts  in  a 
unique  manner  when  subjected  to  pressure.  For  each  addi- 
tional atmosphere  of  pressure — a  weight  amounting  to  about 
fifteen  pounds  to  the  square  inch — the  temperature  at 
which  the  ice  will  melt  is  lowered  to  the  amount  of  sixteen 
thousandths  of  a  degree  centigrade.  If  we  take  a  piece  of 
ice  at  the  temperature  of  freezing  and  put  upon  it  a  suffi- 
cient weight,  we  inevitably  bring  about  a  small  amount  of 
melting.  Where  we  can  examine  the  mass  under  favourable 
conditions,  we  can  see  the  fluid  gather  along  the  lines  of  the 
crystals  or  other  bits  of  which  the  ice  is  composed.  We 
readily  note  this  action  by  bringing  two  pieces  of  ice  to- 
gether with  a  slight  pressure;  when  the  pressure  is  re- 
moved, they  will  adhere.  The  adhesion  is  brought  about 
not  by  any  stickiness  of  the  materials,  for  the  substance 
has  no  such  property.  It  is  accomplished  by  melting  along 
the  line  of  contact,  which  forms  a  film  of  water,  that 
at  once  refreezcs  when  the  pressure  is  withdrawn.    When 


238  OUTLINES  OF  THE  EARTH'S  HISTORY. 

a  firm  snowball  is  made  by  even  pressing  snow,  innumer- 
able similar  adhesions  grow  up  in  the  manner  described. 
The  fact  is  that,  given  ice  at  the  temperature  at  which  it 
ordinarily  forms,  pressure  upon  it  will  necessarily  develop 
melting. 

The  consequences  of  pressure  melting  as  above  de- 
scribed are  in  glaciers  extremely  complicated.  Because  the 
ice  is  built  into  the  glacier  at  a  temperature  considerably 
below  the  freezing  point,  it  requires  a  great  thickness  of 
the  mass  before  the  superincumbent  weight  is  sufficient 
to  bring  about  melting  in  its  lower  parts.  If  we  knew  the 
height  at  which  a  thermometer  would  have  stood  in  the 
surface  ice  of  the  ancient  glacier  which  covered  the  north- 
ern part  of  North  America,  we  could  with  some  accuracy 
compute  how  thick  it  must  have  been  before  the  effect 
of  pressure  alone  would  have  brought  about  melting;  but 
even  then  we  should  have  to  reckon  the  temperature  de- 
rived from  the  grinding  of  the  ice  over  the  floor  and  the 
crushing  of  rocks  there  effected,  as  well  as  the  heat  which 
is  constantly  though  slowly  coming  forth  from  the  earth's 
interior.  The  result  is  that  we  can  only  say  that  at  some 
depth,  probably  less  than  a  mile,  the  slowly  accumulating 
ice  would  acquire  such  a  temperature  that,  subjected  to 
the  weight  above  it,  the  material  next  the  bottom  would 
become  molten,  or  at  least  converted  into  a  sludgelike 
state,  in  which  it  could  not  rub  against  the  bottom,  or 
move  stones  in  the  manner  of  ordinary  glaciers. 

As  fast  as  the  ice  assumed  this  liquid  or  softened  state, 
it  would  be  squeezed  out  toward  the  region  where,  because 
of  the  thinning  of  the  glacier,  it  would  enter  a  field  where 
pressure  melting  did  not  occur.  It  would  then  resume 
the  solid  state,  and  thence  journey  to  the  margin  of  the 
ice  in  the  ordinary  manner.  We  thus  can  imagine  how  such 
a  glacier  as  occupied  the  northern  part  of  this  continent 
could  have  moved  from  the  central  parts  toward  its  periph- 
ery, as  we  can  not  do  if  we  assume  that  the  glacier  every- 
where lay  upon  the  bed  rock.    There  is  no  slope  from  Lake 


GLACIERS.  239 

Erie  to  the  Ohio  Eiver  at  Cincinnati.  Knowing  that  the 
ice  moved  down  this  line,  there  are  but  two  methods  of 
accounting  for  its  motion:  either  the  slope  of  the  upper 
surface  to  the  northward  was  so  steep  that  the  mass  would 
have  been  thus  urged  down,  the  upper  parts  dragging  the 
bottom  along  with  them,  or  the  ice  sheet  for  the  greater 
part  of  its  extent  rested  upon  pressure-molten  water,  or 
sludge  ice,  which  was  easily  squeezed  out  toward  the  front. 
The  first  supposition  appears  inadmissible,  for  the  reason 
that  the  ice  would  have  to  be  many  miles  deep  at  Hudson 
Bay  in  order  that  its  upper  surface  should  have  slope 
enough  to  overcome  the  rigidity  of  the  material  and  bring 
about  the  movement.  We  know  that  any  such  depth  is 
not  supposable. 

The  recent  studies  in  Greenland  supply  us  with  strong 
corroborative  evidence  for  the  support  of  the  view  which 
is  here  urged.  The  wide  central  field  of  that  area,  where 
the  ice  has  an  exceeding  slight  declivity,  and  is  unruptured 
by  crevices,  can  not  be  explained  except  on  the  supposition 
that  it  rests  on  pressure-molten  water.  The  thinner  sec- 
tion next  the  shore,  where  the  glacier  is  broken  up  by 
those  irregular  movements  which  its  wrestle  with  the  bot- 
tom inevitably  induces,  shows  that  there  it  is  in  contact 
with  the  bed  rock,  for  it  behaves  exactly  as  do  the  valley 
glaciers  of  like  thickness. 

The  view  above  suggested  as  to  the  condition  of  conti- 
nental glaciers  enables  us  to  explain  not  only  their  move- 
ments, but  the  relatively  slight  amount  of  wearing  which 
they  brought  about  on  the  lands  they  occupied.  Beginning 
to  develop  in  mountain  regions,  or  near  the  poles  on  the 
lowlands,  these  sheets,  as  soon  as  they  attained  the  thick- 
ness where  the  ice  at  their  bottom  became  molten,  would 
rapidly  advance  for  great  distances  until  they  attained 
districts  where  the  melting  exceeded  the  supply  of  frozen 
material.  In  this  excursion  only  the  marginal  portion 
of  the  glacier  would  do  erosive  work.  This  would  evidently 
be  continued  for  the  greatest  amount  of  time  near  the  front 


240  OUTLINES  OF  THE  EARTH'S  HISTORY. 

or  outer  rim  of  the  ice  field,  for  there,  we  may  presume, 
that  for  the  longest  time  the  cutting  rim  would  rest  upon 
the  bed  rock  of  the  country.  As  the  ice  receded,  this  rim 
would  fall  back;  thus  in  the  retreat  as  in  the  advance  the 
whole  of  the  field  would  be  subjected  to  a  certain  amount 
of  erosion.  On  this  supposition  we  should  expect  to  find 
that  the  front  of  a  continental  glacier,  fed  with  pressure- 
molten  water  from  all  its  interior  district,  which  became 
converted  into  ice,  would  attain  much  warmer  regions  than 
the  valley  streams,  where  all  the  flow  took  place  in  the 
state  of  ice,  and,  furthermore,  that  the  speed  of  the  going 
on  the  margin  would  be  much  more  rapid  than  in  the 
Alpine  streams.  These  suppositions  are  well  borne  out  by 
the  study  of  existing  continental  ice  sheets,  which  move 
with  singular  rapidity  at  their  fronts,  and  by  the  ancient 
glaciers,  which  evidently  extended  into  rather  warm  fields. 
Thus,  when  the  ice  front  lay  at  the  site  of  Cincinnati,  at 
six  hundred  feet  above  the  sea,  there  were  no  glaciers  in 
the  mountains  of  North  Carolina,  though  those  rise  more 
than  five  thousand  feet  higher  in  the  air,  and  are  less  than 
two  hundred  miles  farther  south.  It  is  therefore  evident 
that  the  continental  glacier  at  this  time  pushed  southward 
into  a  comparatively  warm  country  in  a  way  that  no  stream 
moving  in  the  manner  of  a  valley  glacier  could  possibly 
have  done. 

The  continental  glaciers  manage  in  many  cases  to  con- 
vey detritus  from  a  great  distance.  Thus,  when  the  ice 
sheet  advanced  southwardly  from  the  regions  north  of  the 
Great  Lakes,  they  conveyed  quantities  of  the  dehris  from 
that  section  as  far  south  as  the  Ohio  Eiver.  In  part  this 
rubbish  was  dragged  forward  by  the  ice  as  the  sheet  ad- 
vanced; in  part  it  was  urged  onward  by  the  streams  of 
liquid  water  formed  by  the  ordinary  process  of  ice  melt- 
ing. Such  subglacial  rivers  appear  to  have  been  formed 
along  the  margins  of  all  the  great  glaciers.  We  can  some- 
times trace  their  course  by  the  excavation  which  they  have 
made,  but  more  commonly  by  the  long  ridges  of  stratified 


I' 


\ 


\ 


\ 


I 


'^ 


GLACIERS.  211 

sand  and  gravel  which  were  packed  into  the  caverns  ex- 
cavated by  these  subglacial  rivers,  which  are  known  to 
glacialists  as  eshers,  or  as  serpent  kanies.  In  many  cases 
we  can  trace  where  these  streams  flowed  up  stream  in  the 
old  river  valleys  until  they  discharged  over  their  head 
waters.  Thus  in  tlie  valley  of  the  Genesee,  which  now  flows 
from  Pennsylvania,  where  it  heads  against  the  tributaries  of 
the  Ohio  and  Susquehanna,  to  Lake  Ontario,  there  was 
during  the  Glacial  epoch  a  considerable  river  which  dis- 
charged its  waters  into  those  of  the  Oliio  and  the  Susque- 
hanna over  the  falls  at  the  head  of  its  course. 

The  efl'ect  of  widespread  glacial  action  on  a  country 
such  as  North  America  appears  to  have  been,  in  the  first 
place,  to  disturb  the  attitude  of  the  land  by  bearing  down 
portions  of  its  surface,  a  process  which  led  to  the  uprising 
of  other  parts  which  lay  beyond  the  realm  of  the  ice. 
Within  the  field  of  glaciation,  so  far  as  the  ice  rested  bodily 
on  the  surface,  the  rocks  were  rapidly  worn  away.  A  great 
deal  of  the  dehris  was  ground  to  fine  powder,  and  went  far 
with  the  w^aters  of  the  under-running  streams.  A  large 
part  was  entangled  in  the  ice,  and  moved  forward  toward 
the  front  of  the  glacier,  where  it  was  cither  dropped  at 
the  margin  or,  during  the  recession  of  the  glacier,  was 
laid  upon  the  surface  as  the  ice  melted  away.  The  result 
of  this  erosion  and  transportation  has  been  to  change  the 
conditions  of  the  surface  both  as  regards  soil  and  drainage. 
As  the  reader  has  doubtless  perceived,  ordinary  soil  is,  out- 
side of  the  river  valleys,  derived  from  the  rock  beneath 
where  it  lies.  In  glaciated  districts  the  material  is  com- 
monly brought  from  a  considerable  distance,  often  from 
miles  away.  These  ice-made  soils  are  rarely  very  fertile, 
but  they  commonly  have  a  great  endurance  for  tillage,  and 
this  for  the  reason  that  the  earth  is  refreshed  by  the  decay 
of  the  pebbles  which  they  contain.  Moreover,  while  the 
tillable  earth  of  other  regions  usually  has  a  limited  depth, 
verging  downward  into  the  semisoil  or  subsoil  which  rep- 
resent the  little  changed  bed  rocks,  glacial  deposits  can 


2i2  OUTLINES  OF  THE  EARTH'S  HISTORY. 

generally  be  ploughed  as  deeply  as  may  prove  desir- 
able. 

The  drainage  of  a  country  recently  affected  by  glaciers 
is  always  imperfect.  Owing  to  the  irregular  erosion  of  the 
bed  rocks,  and  to  the  yet  more  irregular  deposition  of  the 
detritus,  there  are  very  numerous  lakes  which  are  only 
slowly  filled  up  or  by  erosion  provided  with  drainage  chan- 
nels. Though  several  thousand  years  have  passed  by  since 
the  ice  disappeared  from  North  America,  the  greater  part 
of  the  area  of  these  fresh-water  basins  remains,  the  greater 
number  of  them,  mostly  those  of  small  size,  have  become 
closed. 

Where  an  ice  stream  descends  into  the  sea  or  into  a 
large  lake,  the  depth  of  which  is  about  as  great  as  the  ice 
is  thick,  the  relative  lightness  of  the  ice  tends  to  make  it 
float,  and  it  shortly  breaks  off  from  the  parent  mass,  form- 
ing an  iceberg.  Where,  as  is  generally  the  case  in  those 
glaciers  which  enter  the  ocean,  a  current  sweeps  by  the 
place  where  the  berg  is  formed,  it  may  enter  upon  a  jour- 
ney which  may  carry  the  mass  thousands  of  miles  from  its 
origin.  The  bergs  separated  from  the  Greenland  glaciers, 
and  from  those  about  the  south  pole,  are  often  of  very 
great  size;  sometimes,  indeed,  they  are  some  thousand  feet 
in  thickness,  and  have  a  length  of  several  miles.  It  often 
happens  that  these  bergs  are  formed  of  ice,  which  contains 
in  its  lower  part  a  large  amount  of  rock  debris.  As  the 
submerged  portion  of  the  glacier  melts  in  the  sea  water, 
these  stones  are  gradually  dropped  to  the  bottom,  so  that 
the  cargo  of  one  berg  may  be  strewed  along  a  line  many 
hundred  miles  in  length.  It  occasionally  happens  that  the 
ice  mass  melts  more  slowly  in  those  parts  which  are  in  the 
air  than  in  its  under-water  portions.  It  thus  becomes  top- 
heavy  and  overturns,  in  which  case  such  stony  matter  as 
remains  attains  a  position  where  it  may  be  conveyed  for  a 
greater  distance  than  if  the  glacier  were  not  capsized.  It 
is  likely,  indeed,  that  now  and  then  fragments  of  rock  from 
Greenland  are  dropped  on  the  ocean  floor  in  the  part  of  the 


GLACIERS.  213 

Atlantic  which  is  traversed  by  steamers  between  our  At- 
lantic ports  and  Great  Britain. 

Except  for  the  risks  which  they  bring  to  navigators, 
icebergs  have  no  considerable  importance.  It  is  true  they 
somewhat  affect  tne  temperature  of  sea  and  air,  and  they 
also  serve  to  jonvey  fragments  of  stone  far  out  to  sea  in 
a  way  that  no  other  agent  can  effect;  but,  on  the  whole, 
their  influence  on  the  conditions  of  the  earth  is  incon- 
sider^ible. 

Icebergs  in  certain  cases  afford  interesting  indices  as 
to  the  motion  of  oceanic  currents,  which,  though  moving 
swiftly  at  a  depth  below  the  surface,  do  not  manifest  them- 
selves on  the  plain  of  the  sea.  Thus  in  the  region  about 
Greenland,  particularly  in  Davis  Strait,  bergs  have  been 
seen  forcing  their  way  southward  at  considerable  speed 
through  ordinary  surface  ice,  which  was  either  at  rest  or 
moving  in  the  opposite  direction.  The  train  of  these  bergs, 
which  moves  upward  from  the  south  polar  continent,  west 
of  Patagonia,  indicates  also  in  a  very  emphatic  way  the 
existence  of  a  very  strong  northward-setting  current  in 
that  part  of  the  ocean. 

We  have  now  to  consider  the  causes  which  could  bring 
about  such  great  extension^  of  the  ice  sheet  as  occurred 
in  the  last  Glacial  period.  Here  again  we  are  upon  the 
confines  of  geological  knowledge,  and  in  a  field  where  there 
are  no  well-cleared  ways  for  the  understanding.  In  facing 
this  problem,  we  should  first  note  that  those  who  are  of  the 
opinion  that  a  Glacial  period  means  a  very  cold  climate  in 
the  regions  where  the  ice  attained  its  extension  are  prob- 
ably in  error.  Natural  as  it  may  seem  to  look  for  exceed- 
ing cold  as  the  cause  of  glaciation,  the  facts  show  us  that 
we  can  not  hold  this  view.  In  Siberia  and  in  the  parts  of 
North  America  bordering  on  the  Arctic  Sea  the  average 
cold  is  so  intense  that  the  ground  is  permanently  frozen 
— as  it  is,  for  instance,  in  the  Klondike  district — to  the 
depth  of  hundreds  of  feet,  only  the  surface  thawing  out 


24:4:  OUTLINES  OF  THE  EARTH'S  HISTORY. 

during  the  warm  summers.  All  this  region  is  cold  enough 
for  glaciers,  but  there  is  not  sufficient  snowfall  to  maintain 
them.  On  the  other  hand,  in  Greenland,  and  in  a  less 
though  conspicuous  degree  in  Scandinavia,  Avhere  the 
waters  of  the  North  Atlantic  somewhat  diminish  the  rigour 
of  the  cold,  and  at  the  same  time  bring  about  a  more 
abundant  snowfall,  the  two  actions  being  intimately  re- 
lated, we  have  very  extensive  glaciers.  Such  facts,  which 
could  be  very  much  extended,  make  it  clear  that  the  climate 
of  glacial  periods  must  have  been  characterized  by  a  great 
snowfall,  and  not  by  the  most  intense  cold. 

It  is  evident  that  what  would  be  necessary  again  to  en- 
velop the  boreal  parts  of  North  America  with  a  glacial 
sheet  would  not  be  a  considerable  decrease  of  heat,  but  an 
increase  in  the  winter's  contribution  of  frozen  water.  Even 
if  the  heat  released  by  this  snowfall  elevated  the  average 
temperature  of  the  winter,  as  it  doubtless  would  in  a  con- 
siderable measure,  it  would  not  melt  off  the  snow.  That 
snowfall  tends  to  warm  the  air  by  setting  free  the  heat 
which  was  engaged  in  keeping  the  water  in  a  state  of 
vapour  is  familiarly  shown  by  the  warming  which  attends 
an  ordinary  snowstorm.  Even  if  the  fall  begin  with  a  tem- 
perature of  about  0°  Fahr.,  the  air  is  pretty  sure  to  rise  to 
near  the  freezing  point. 

It  is  evident  that  no  great  change  of  temperature  is 
required  in  order  to  bring  about  a  very  considerable  in- 
crease in  the  amount  of  snowfall.  In  the  ordinary  suc- 
cession of  seasons  we  often  note  the  occurrence  of  winters 
during  which  the  precipitation  of  snow  is  much  above  the 
average,  though  it  can  not  be  explained  by  a  considerable 
climatal  change.  We  have  to  account  for  these  departures 
from  the  normal  weather  by  supposing  that  the  atmospheric 
currents  bring  in  more  than  the  usual  amount  of  moisture 
from  the  sea  during  the  period  when  great  falls  of  snow 
occur.  In  fact,  in  explaining  variations  in  the  humidity 
of  the  land,  whether  those  of  a  constant  nature  or  those 
that  are  to  be  termed  accidental;  we  have  always  to  look 


GLACIERS.  245 

to  those  features  which  determine  the  importation  of 
vapour  from  the  great  field  of  the  ocean  where  it  enters 
the  air.  We  should  furthermore  note  that  these  peculiari- 
ties of  climate  are  dependent  upon  rather,  slight  geo- 
graphic accidents.  Thus  the  snowfall  of  northern  Europe, 
which  serves  to  maintain  the  glaciation  of  that  region,  and, 
curiously  enough,  in  some  measure  its  general  warmth, 
depends  upon  the  movement  of  the  Gulf  Stream  from  the 
tropics  to  high  latitudes.  If  by  any  geographical  change, 
such  as  would  occur  if  Central  America  were  lowered  so 
as  to  make  a  free  passage  for  its  waters  to  the  westward, 
the  glaciers  of  Greenland  and  of  Scandinavia  would  dis- 
appear, and  at  the  same  time  the  temperature  of  those 
would  be  greatly  lowered.  Thus  the  most  evident  cause  of 
glaciation  must  be  sought  in  those  alterations  of  the  land 
which  affect  the  movement  of  the  oceanic  currents. 

Applying  this  principle  to  the  northern  hemisphere, 
we  can  in  a  way  imagine  a  change  which  would  probably 
bring  about  a  return  of  such  an  ice  period  as  that  from 
which  the  boreal  realm  is  now  escaping.  Let  us  suppose 
that  the  region  of  not  very  high  land  about  Bering 
Strait  should  sink  down  so  as  to  afford  the  Kuro  Siwo,  or 
North  Pacific  equivalent  of  our  Gulf  Stream,  an  oppor- 
tunity to  enter  the  Arctic  Sea  with  something  like  the 
freedom  with  which  the  North  Atlantic  current  is  allowed 
to  penetrate  to  high  latitudes.  It  seems  likely  that  this 
Pacific  current,  which  in  volume  and  warmth  is  com- 
parable to  that  of  the  Atlantic,  would  so  far  elevate  the 
temperature  of  the  arctic  waters  that  their  wide  field 
would  be  the  seat  of  a  great  evaporation.  Noting  once 
again  the  fact  that  the  Greenland  glaciers,  as  well  as  those 
of  Norway,  are  supplied  from  seas  warmed  by  the  Gulf 
Stream,  we  should  expect  the  result  of  this  change  would 
be  to  develop  similar  ice  fields  on  all  the  lands  near  that 
ocean. 

Applying  the  data  gathered  by  Dr.  Croll  for  the  Gulf 
Stream,  it  seems  likely  that  the  average  annual  temperature 


246  OUTLINES  OF  THE  EARTH'S  HISTORY. 

induced  in  the  Arctic  Sea  by  the  free  entrance  of  the  Japan 
current  would  be  between  20°  and  30°  Fahr.  This  would 
convert  this  wide  realm  of  waters  into  a  field  of  great 
evaporation,  vastly  increasing  the  annual  precipitation. 
It  seems  also  certain  that  the  greater  part  of  this  precipita- 
tion would  be  in  the  form  of  snow.  It  appears  to  the 
writer  that  this  cause  alone  may  be  sufficient  to  account 
for  the  last  Glacial  period  in  the  northern  hemisphere. 
As  to  the  probability  that  the  region  about  Bering  Strait 
may  have  been  lowered  in  the  manner  required  by  this 
view,  it  may  be  said  that  recent  studies  on  the  region  about 
Mount  St.  Elias  show  that  during  or  just  after  the  ice 
epoch  the  shores  in  that  portion  of  Alaska  were  at  least 
four  thousand  feet  lower  than  at  present.  As  this  is  but 
a  little  way  from  the  land  which  we  should  have  to  suppose 
to  be  lowered  in  order  to  admit  the  Japan  current,  we 
could  fairly  conclude  that  the  required  change  occurred. 
As  for  the  cause  of  the  land  movement,  geologists  are  still 
in  doubt.  They  know,  however,  that  the  attitudes  of  the 
land  are  exceedingly  unstable,  and  that  the  shores  rarely 
for  any  considerable  time  maintain  their  position.  It  is 
probable  that  these  swayings  of  the  earth's  surface  are  due 
to  ever-changing  combinations  of  the  weight  in  different 
parts  of  the  crust  and  the  strains  arising  from  the  con- 
traction of  its  inner  parts. 

In  the  larger  operations  of  Nature  the  effects  which 
we  behold,  however  simple,  are  rarely  the  products  of  a 
single  cause.  In  fact,  there  are  few  actions  so  limited  that 
they  can  fairly  be  referred  to  one  influence.  It  is  there- 
fore proper  to  state  that  there  are  many  other  actions 
besides  those  above  noted  which  probably  enter  into  those 
complicated  equations  which  determine  the  climatal  con- 
ditions of  the  earth.  To  have  these  would  carry  us  into 
difficult  and  speculative  inquiries. 

As  before  remarked,  all  the  regions  which  have  been 
subjected  to  glaciation  are  still  each  year  brought  tem- 
porarily into  the  glacial  state.     This  fact  serves  to  show 


GLACIERS.  247 

us  that  the  changes  necessary  to  produce  great  ice  sheets 
are  not  necessarily  of  a  startling  nature,  however  great  the 
consequences  may  be.  Assuming,  then,  that  relatively 
slight  alterations  of  climate  may  cause  the  ice  sheet  to 
come  and  go,  we  may  say  that  all  the  influences  which 
have  been  suggested  by  the  students  of  glaciation,  and 
various  other  slighter  causes  which  can  not  be  here  noted, 
may  have  co-operated  to  produce  the  peculiar  result.  In 
this  equation  geographic  change  has  affected  the  course 
of  the  ocean  currents,  and  has  probably  been  the  most 
influential,  or  at  least  the  commonest,  cause  to  which  we 
must  attribute  the  extension  of  ice  sheets.  Next,  altera- 
tions of  the  solar  heat  may  be  looked  to  as  a  change-bring- 
ing action;  unfortunately,  however,  we  have  no  direct  evi- 
dence that  this  is  an  efficient  cause.  Thirdly,  the  varia- 
tions in  the  eccentricity  of  the  earth's  orbit,  combined  with 
the  precession  of  the  equinoxes  and  the  rotation  of  the 
apsides,  may  be  regarded  as  operative.  The  last  of  all, 
changes  in  the  constitution  of  the  atmosphere,  have  to  be 
taken  into  account.  To  these  must  be  added,  as  before 
remarked,  many  less  important  actions  which  influence 
this  marvellously  delicate  machine,  the  work  of  which  is 
expressed  in  the  phenomena  assembled  under  the  name  of 
climate. 

Evidence  is  slowly  accumulating  which  serves  to  show 
that  glacial  periods  of  greater  or  less  importance  have  been 
of  frequent  occurrence  at  all  stages  in  the  history  of  the 
earth  of  which  we  have  a  distinct  record.  As  these  acci- 
dents write  their  history  upon  the  ground  alone,  and  in 
a  way  impermanently,  it  is  difficult  to  trace  the  ice  times 
of  ancient  geological  periods.  The  scratches  on  the  bed 
rocks,  and  the  accumulations  of  detritus  formed  as  the  ice 
disappeared,  have  alike  been  worn  away  by  the  agents  of 
decay.  Nevertheless,  we  can  trace  here  and  there  in  the 
older  strata  accumulations  of  pebbly  matter  often  con- 
taining large  boulders,  which  clearly  were  shaped  and 
brought  together  by  glacial  action.  These  are  found  in 
17 


248  OUTLINES  OP  THE  EAIITH'S  HISTORY. 

some  instances  far  south  of  the  region  occupied  by  the 
glaciers  during  the  last  ice  epoch.  They  occur  in  rocks 
of  the  Cambrian  or  Silurian  age  in  eastern  Tennessee  and 
western  North  Carolina;  they  are  also  found  in  India  be- 
yond the  limits  to  which  glaciers  have  attained  in  modern 
times. 

In  closing  this  inadequate  account  of  glacial  action, 
a  story  which  for  its  complete  telling  would  require  many 
volumes,  it  is  well  for  the  reader  to  consider  once  again 
how  slight  are  the  changes  of  climate  which  may  alter- 
nately withdraw  large  parts  of  the  land  from  the  uses  of 
life,  and  again  quickly  restore  the  fields  to  the  service 
of  plants  and  animals.  He  may  well  imagine  that  these 
changes,  by  driving  living  creatures  to  and  fro,  profoundly 
affect  the  history  of  their  development.  This  matter  will 
be  dealt  with  in  the  volume  concerning  the  history  of 
organic  beings. 

When  the  ice  went  oif  from  the  northern  part  of  this 
continent,  the  surface  of  the  country,  which  had  been 
borne  down  by  the  weight  of  the  glacier,  still  remained  de- 
pressed to  a  considerable  depth  below  the  level  of  the  sea, 
the  depression  varying  from  somewhere  about  one  hun- 
dred feet  in  southern  New  England  to  a  thousand  feet  or 
more  in  high  latitudes.  Over  this  region,  which  lay  be- 
neath the  level  of  the  sea,  the  glacier,  when  it  became  thin 
enough  to  float,  was  doubtless  broken  up  into  icebergs,  in 
the  manner  which  we  now  behold  along  the  coast  of 
Greenland.  Where  the  shore  was  swept  by  a  strong  cur- 
rent, these  bergs  doubtless  drifted  away;  but  along  the 
most  of  the  coast  line  they  appear  to  have  lain  thickly 
grouped  next  the  shores,  gradually  delivering  their  loads 
of  stones  and  finer  debris  to  the  bottom.  These  masses  of 
floating  ice  in  many  cases  seem  to  have  prevented  the  sea 
waves  from  attaining  the  shore,  and  thus  hindered  the 
formation  of  those  beaches  which  in  their  present  elevated 
condition  enable  us  to  interpret  the  old  position  of  the  sea 
along  coast  lines  which  have  been  recently  elevated.    Here 


GLACIERS.  249 

and  there,  however,  from  New  Jersey  to  Greenland,  we 
find  bits  of  these  ancient  shores  which  clearly  tell  the 
story  of  that  down-sinking  of  the  land  beneath  the  burden 
of  the  ice  which  is  such  an  instructive  feature  in  the  his- 
tory of  that  period. 


CHAPTER  VII. 

THE  WOEK  OF  UNDERGROUND  WATER. 

We  have  already  noted  two  means  by  which  water  finds 
its  way  underground.  The  simplest  and  largest  method  by 
which  this  action  is  effected  is  by  building  in  the  fluid 
as  the  grains  of  the  rock  are  laid  down  on  the  floors  of 
seas  or  lakes.  The  water  thus  imprisoned  is  firmly  in- 
closed in  the  interstices  of  the  stone,  it  in  time  takes  up 
into  its  mass  a  certain  amount  of  the  mineral  materials 
which  are  contained  in  the  deep-buried  rocks.  The  other 
portion  of  the  ground  water — that  with  which  we  are  now 
to  be  specially  concerned — arises  from  the  rain  which 
descends  into  the  crevices  of  the  earth;  it  is  therefore  pe- 
culiar to  the  lands.  For  ^convenience  we  shall  term  the 
original  embedded  fluid  roclc  water,  and  that  which  origi- 
nates from  the  rain  crevice  water,  the  two  forming  the  mass 
of  the  earth  water. 

The  crevice  water  of  the  earth,  although  forming  at 
no  time  more  than  a  very  small  fraction  of  the  hidden 
fluid,  is  an  exceedingly  potent  geological  agent,  doing  work 
which,  though  unseen,  yet  affords  the  very  foundations  on 
which  rest  the  life  alike  of  land  and  sea.  When  this  water 
enters  the  earth,  though  it  is  purified  of  all  mineral  mate- 
rials, it  has  already  begun  to  acquire  a  share  of  a  gaseous 
substance,  carbonic  acid,  or,  as  chemists  now  term  it, 
carbon  dioxide,  which  enables  tlie  fluid  to  begin  its  role 
of  marvellous  activities.  In  its  descent  as  rain,  probably 
even  before  it  was  gathered  in  drops  in  the  cloud  realm^ 

350 


THE  WORK  OF  UNDERGROUND  WATER.    251 

the  water  absorbs  a  certain  portion  of  this  gas  from  the 
atmosphere.  Entering  the  realm  of  the  soil,  where  the 
decaying  organic  matter  plentifully  gives  forth  carbon 
dioxide,  a  further  store  of  the  gas  is  acquired.  At  the 
ordinary  pressure  of  the  air,  water  may  take  in  many  times 
its  bulk  of  the  gas. 

The  immediate  effect  of  carbonic  acid  when  it  is  ab- 
sorbed by  water  is  greatly  to  increase  the  capacity  which 
that  fluid  has  for  taking  mineral  matters  into  solution. 
When  charged  with  this  gas,  in  the  measure  in  which  it 
may  be  in  the  soil,  water  is  able  to  dissolve  about  fifty 
times  as  much  limestone  as  it  can  in  its  perfectly  pure 
form  take  up.  A  familiar  instance  of  this  peculiar  capacity 
which  the  gas  gives  may  often  be  seen  where  the  water 
from  a  soda-water  fountain  drips  upon  the  marble  slab 
beneath.  In  a  few  years  this  slab  will  be  considerably 
corroded,  though  pure  water  would  in  the  same  time  have 
had  no  effect  upon  it. 

The  first  and  by  far  the  most  important  effect  of  crevice 
water  is  exercised  upon  the  soil,  which  is  at  once  the  prod- 
uct of  this  action,  and  the  laboratory  where  the  larger  part 
of  the  work  is  done.  Penetrating  between  the  grains  of 
the  detrital  covering,  held  in  large  quantities  in  the  coat- 
ing, and  continually  in  slow  motion,  the  gas-charged  water 
takes  a  host  of  substances  into  solution,  and  brings  them 
into  a  condition  where  they  may  react  upon  each  other 
in  the  chemical  manner.  These  materials  are  constantly 
being  offered  to  the  roots  of  plants  and  brought  in  con- 
tact with  the  underlying  rock  which  has  not  passed  into  the 
state  of  soil.  The  changes  induced  in  this  stony  matter 
lead  to  its  breaking  up,  or  at  least  to  its  softening  to  the 
point  where  the  roots  can  penetrate  it  and  complete  its 
destruction.  Thus  it  comes  about  that  the  water  which 
to  a  great  extent  divides  the  rocks  into  the  state  of  soil, 
which  is  continually  wearing  away  the  material  on  the  sur- 
face, or  leaching  it  out  through  the  springs,  is  also  at  work 
in  restoring  the  layer  from  beneath. 


252  OUTLINES  OP  THE  EARTH'S  HISTORY. 

The  greater  part  of  the  water  which  enters  the  soil 
does  not  penetrate  to  any  great  depth  in  the  underlying 
rocks,  but  finds  its  way  to  the  surface  after  no  long  journey 
in  the  form  of  small  springs.  Generally  these  superficial 
springs  do  not  emerge  through  distinct  channels,  but  move, 
though  slowly,  in  a  massive  way  down  the  slopes  until 
they  enter  a  water  course.  Along  the  banks  of  any  river, 
however  small,  or  along  the  shores  of  the  sea,  a  pit  a  few 
inches  deep  just  above  the  level  of  the  water  will  be  quickly 
filled  by  a  flow  from  this  sheet  which  underlies  the  earth. 
At  a  distance  from  the  stream  this  sheet  spring  is  in  con- 
tact with  the  bed  rocks,  and  may  be  many  feet  below  the 
surface,  but  it  comes  to  the  level  of  the  river  or  the  sea 
near  their  margins.  Here  and  there  the  shape  of  the  bed 
rocks,  being  like  converging  house  roofs,  causes  the  super- 
ficial springs  to  form  small  pipelike  channels  for  the  escape 
of  their  gathered  waters,  and  the  flow  emerges  at  a  definite 
point.  Almost  all  these  sources  of  considerable  flow  are 
due  to  the  action  of  the  water  on  the  underlying  rock, 
where  we  shall  now  follow  that  portion  of  the  crevice 
water  which  penetrates  deeply  into  the  earth. 

Almost  all  rocks,  however  firm  they  may  appear  to 
be,  are  divided  by  crevices  which  extend  from  the  soil 
level  it  may  be  to  the  depths  of  thousands  of  feet.  These 
rents  are  in  part  due  to  the  strains  of  mountain-building, 
which  tend  to  disrupt  the  firmest  stone,  leaving  open  frac- 
tures. They  are  also  formed  in  other  ways,  as  by  the  im- 
perfectly understood  agencies  which  produce  joint  planes. 
It  often  happens  that  where  rocks  are  highly  tilted  water 
finds  its  way  downward  between  the  layers,  which  are  im- 
perfectly soldered  together,  or  a  bed  of  coarse  material, 
such  as  sandstone  or  conglomerate,  may  afford  an  easy  way 
by  which  the  water  may  descend  for  miles  beneath  the 
surface.  Passing  through  rocks  which  are  not  readily  sol- 
uble, the  water,  already  to  a  great  extent  supplied  with 
mineral  matter  by  its  journey  through  the  soil,  may  not 
do  much  excavating  work,  and  even  after  a  long  time  may 


THE   WORK  OF  UNDEEGROUND  WATER.         253 

only  slightly  enlarge  the  spaces  in  which  it  may  be  stored 
or  the  channels  by  which  it  discharges  to  the  surface. 
Hence  it  comes  about  that  in  many  countries,  even  where 
the  waters  penetrate  deeply,  they  do  not  afford  large 
springs.  It  is  otherwise  where  the  crevice  waters  enter 
limestones  composed  of  materials  which  are  readily  dis- 
solved. In  such  places  we  find  the  rain  so  readily  entering 
the  underlying  rock  that  no  part  of  the  fall  goes  at  once 
to  the  brooks,  but  all  has  a  long  underground  journey. 

In  any  limestone  district  where  the  beds  of  the  mate- 
rial are  thick  and  tolerably  pure — as,  for  instance,  in  the 
cavern  district  of  southern  Kentucky — the  traveller  who 
enters  the  region  notes  at  once  that  the  usual  small  streams 
which  in  every  region  of  considerable  rainfall  he  is  accus- 
tomed to  see  intersecting  the  surface  of  the  country  are 
entirely  absent.  In  their  place  he  notes  everywhere  pitlike 
depressions  of  bowl-shaped  form,  the  sink  holes  to  which 
we  have  already  adverted.  Through  the  openings  in  the 
bottom  of  these  the  rain  waters  descend  into  the  depths 
of  the  earth.  Although  the  most  of  these  depressions  have 
but  small  openings  in  their  bottom,  now  and  then  one 
occurs  with  a  vertical  shaft  sufficiently  large  to  permit  the 
explorer  to  descend  into  it,  though  he  needs  to  be  lowered 
down  in  the  manner  of  a  miner  who  is  entering  a  shaft. 
In  fact,  the  journey  is  nearly  always  one  of  some  hazard; 
it  should  not  be  undertaken  save  with  many  precautions 
to  insure  safety. 

When  one  is  lowered  away  through  an  open  sink  hole, 
though  the  descent  may  at  first  be  somewhat  tortuous,  the 
explorer  soon  finds  himself  swinging  freely  in  the  air,  it 
may  be  at  a  point  some  hundred  feet  above  the  base  of  the 
bottle-shaped  shaft  or  dome  into  which  he  has  entered. 
Commonly  the  neck  of  the  bottle  is  formed  where  the 
water  has  worked  its  way  through  a  rather  sandy  limestone, 
a  rock  which  was  not  readily  dissolved  by  the  water.  In 
the  pure  and  therefore  easily  cut  limestone  layers  the  cav- 
ity rapidly  expands  until  the  light  of  the  lantern  may 


254  OUTLINES  OF  THE  EARTH'S  HISTORY. 

not  disclose  its  walls.  Farther  down  there  is  apt  to  be  a 
shelf  composed  of  another  impure  limestone,  which  ex- 
tends off  near  the  middle  of  the  shaft.  If  the  explorer 
can  land  upon  this  shelf,  he  is  sure  to  find  that  from  this 
imperfect  floor  the  cavern  extends  off  in  one  or  more  hori- 
zontal galleries,  which  he  may  follow  for  a  great  distance 
until  he  comes  to  the  point  where  there  is  again  a  well- 
like opening  through  the  hard  layer,  with  another  dome- 
shaped  base  beneath.  Returning  to  the  main  shaft,  the 
explorer  may  continue  his  descent  until  he  attains  the 
base  of  this  vertical  section  of  the  cave,  where  he  is  likely 
to  find  himself  delivered  in  a  pool  of  water  of  no  great 
depth,  the  bottom  of  which  is  occupied  by  a  quantity  of 
small,  hard  stones  of  a  flinty  nature,  which  have  evidently 
come  from  the  upper  parts  of  the  cavern.  The  close  ob- 
server will  have  noted  that  here  and  there  in  the  limestone 
there  are  flinty  bits,  such  as  those  which  he  finds  in  the 
pool..  From  the  bottom  of  the  dome  a  determined  in- 
quirer can  often  make  his  way  along  the  galleries  which 
lead  from  that  level,  though  it  may  be  after  a  journey  of 
miles  to  the  point  where  he  emerges  from  the  cavern  on 
the  banks  of  an  open-air  river. 

Although  a  journey  by  w^ay  of  the  sink  holes  through 
a  cavern  system  is  to  be  commended  for  the  reason  that 
it  is  the  course  of  the  caverning  waters,  it  is,  on  the  whole, 
best  to  approach  the  cave  through  their  exits  along  the 
banks  of  a  stream  or  through  the  chance  openings  which 
are  here  and  there  made  by  the  falling  in  of  their  roofs. 
One  advantage  of  this  cavity  of  entrance  is  that  we  can  thus 
approach  the  cavern  in  times  of  heavy  rain  when  the  pro- 
cesses which  lead  to  their  construction  are  in  full  activity. 
Coming  in  this  way  to  one  of  the  domes  formed  beneath 
a  sink  hole,  we  may  observe  in  rainy  weather  that  the 
water  falling  down  the  deep  shaft  strikes  the. bottom  with 
great  force;  in  many  of  the  Kentucky  caves  it  falls  from 
a  greater  height  than  Niagara.  At  such  times  the  stones 
in  the  basin  at  the  bottom  of  the  shaft  are  vigorously 


THE  WORK  OF  UNDERGROUND  WATER.         255 

whirled  about,  and  in  their  motion  they  cut  the  rocks  in 
the  bottom  of  the  basin — in  fact,  this  cavity  is  a  great  pot 
hole,  like  those  at  the  base  of  open-air  cascades.  It  is  now 
easy  to  interpret  the  general  principles  which  determine 
the  architecture  of  the  cavern  realm. 

When  it  first  enters  the  earth  all  the  work  which  the 
water  does  in  the  initial  steps  of  cavern  formation  is  ef- 
fected by  solution.  As  the  crevice  enlarges  and  deepens, 
the  stream  acquires  velocity,  and  begins  to  use  the  bits  of 
hard  rock  in  boring.  It  works  downward  in  this  way  by 
the  mixed  mechanical  and  chemical  action  until  it  en- 
counters a  hard  layer.  Then  the  water  creeps  horizontally 
through  the  soft  stratum,  doing  most  of  its  work  by  solu- 
tion, until  it  finds  a  crevice  in  the  floor  through  which 
it  can  excavate  farther  in  the  downward  direction;  so  it 
goes  on  in  the  manner  of  steps  until  it  burrows  channels 
to  the  open  stream.  In  time  the  vertical  fall  under  the 
sink  hole  will  cut  through  the  hard  layer,  when  the  water, 
abandoning  the  first  line  of  exit,  will  develop  another  at 
a  lower  level,  and  so  in  time  it  comes  about  that  there  may 
be  several  stories  of  the  cave,  the  lowest  being  the  last  to 
be  excavated.  Of  the  total  work  thus  done,  only  a  small 
part  is  accomplished  by  the  falling  of  the  water,  acting 
through  the  boring  action  of  its  tools,  the  bits  of  stone 
before  mentioned;  the  principal  part  of  the  task  is  done 
by  the  solvent  action  of  the  carbonated  waters  on  the  lime- 
stone. In  the  system  of  caverns  known  as  the  Mammoth 
Cave,  in  Kentucky,  the  writer  has  estimated  that  at  least 
nine  tenths  of  the  stone  was  removed  in  the  state  of  solu- 
tion. 

When  first  excavated,  the  chambers  of  a  limestone  cav- 
ern have  little  beauty  to  attract  the  eye.  The  curves  of  the 
walls  are  sometimes  graceful,  but  the  aspect  of  the  cham- 
bers, though  in  a  measure  grand,  is  never  charming.  Wlien, 
however,  the  waters  have  ceased  to  carve  the  openings, 
when  they  have  been  drained  away  by  the  formation  of 
channels  on  a  lower  level,  there  commonly  sets  in  a  pro- 


256  OUTLINES  OF  THE  EARTH'S  HISTOEY. 

cess  known  as  stalactitization,  which  transforms  the  scene 
into  one  of  singular  beauty.  We  have  already  noted  the 
fact  that  everywhere  in  ordinary  rocks  there  are  crevices 
through  which  water,  moving  under  the  pressure  of  the 
fluid  which  is  above,  may  find  its  way  slowly  downward. 
In  the  limestone  roofs  of  caverns,  particularly  in  those 
of  the  upper  story,  this  ooze  of  water  passes  through  myri- 
ads of  unseen  fissures  at  a  rate  so  slow  that  it  often  evapo- 
rates in  the  dry  air  without  dropping  to  the  floor.  When 
it  comes  out  of  the  rocks  the  water  is  charged  with  various 
salts  of  lime;  when  it  evaporates  it  leaves  the  material  be- 
hind on  the  roof.  Where  the  outflow  is  so  slight  that  the 
fluid  does  not  gather  into  drops,  it  forms  an  incrustation 
of  limy  matter,  which  often  gathers  in  beautiful  flowerlike 
forms,  or  perhaps  in  the  shape  of  a  sheet  of  alabaster.  Where 
drops  are  formed,  a  small,  pendent  cone  grows  downward 
from  the  ceiling,  over  which  the  water  flows,  and  on  which  it 
evaporates.  This  cone  grows  slowly  downward  until  it  may 
attain  the  floor  of  the  chamber,  which  has  a  height  of  thirty 
feet  or  more.  If  all  the  water  does  not  evaporate,  that 
which  trickles  off  the  apex  of  the  cone,  striking  on  the 
floor,  is  splashed  out  into  a  thin  sheet,  so  that  it  evaporates 
in  a  speedy  manner,  lays  down  its  limestone,  and  thus 
builds  another  and  ruder  cone,  which  grows  upward  toward 
that  which  is  pendent  above  it.  Finally,  they  grow  to- 
gether, enlarge  by  the  process  which  constructed  them, 
until  a  mighty  column  may  be  formed,  sculptured  as  if 
by  the  hands  of  a  fantastic  architect. 

All  the  while  that  subterranean  streams  are  cutting 
the  caverns  downward  the  open-air  rivers  into  which  they 
discharge  are  deepening  their  beds,  and  thereby  preparing 
for  the  construction  of  yet  lower  stojies  of  caves.  These 
open-air  streams  commonly  flow  in  steep-sided,  narrow 
valleys,  which  themselves  were  caves  until  the  galleries  be- 
came so  wide  that  they  could  no  longer  support  the  roof. 
Thus  we  often  find  that  for  a  certain  distance  the  roof 
over  a  large  stream  has  fallen  in,  so  that  the  water  flows  in 


THE  WORK  OF  UNDERGROUND  WATER.    25"? 

the  open  air.  Then  it  will  plunge  under  an  arch  and 
course,  it  may  be,  for  some  miles,  before  it  again  arrives 
at  a  place  where  the  roof  has  disappeared,  or  perhaps  at- 
tains a  field  occupied  by  rocks  of  another  character,  in 


Pig.  13. — Stalactites  and  stalagmites  on  roof  and  floor  of  a  cavern. 
The  arrows  sliow  the  direction  of  the  moving  water. 

which  caverns  were  not  formed.  At  places  these  old  river 
caverns  are  abandoned  by  the  streams,  which  find  other 
courses.  They  form  natural  tunnels,  which  are  not  in- 
frequently of  considerable  length.  One  such  in  south- 
western Virginia  has  been  made  useful  for  a  railway  pass- 
ing from  one  valley  to  another,  thus  sparing  the  expense 
of  a  costly  excavation.  Where  the  remnant  of  the  arch  is 
small,  it  is  commonly  known  as  a  natural  bridge,  of  which 
that  in  Rockbridge  County,  in  Virginia,  is  a  very  noble 


258  OUTLINES  OF  THE  EARTH'S  HISTORY. 

example.  Arches  of  this  sort  are  not  uncommon  in  many 
cavern  countries;  five  such  exist  in  Carter  County,  Ken- 
tucky, a  district  in  the  eastern  part  of  that  State  which 
abounds  in  caverns,  though  none  of  them  are  of  conspicu- 
ous height  or  beauty.* 

At  this  stage  of  his  studies  on  cavern  work  the  student 
will  readily  conceive  that,  as  the  surface  of  the  country 
overlying  the  cave  is  incessantly  wearing  down,  the  upper 
stories  of  the  system  are  continually  disappearing,  while 
new  ones  are  forming  at  the  present  drainage  level  of  the 
country.  In  fact,  the  attentive  eye  can  in  such  a  district 
find  here  and  there  evidences  of  this  progressive  destruc- 
tion. Not  only  do  the  caves  wear  out  from  above,  but 
their  roofs  are  constantly  falling  to  their  floors,  a  process 
which  is  greatly  aided  by  the  growth  of  stalactites.  Form- 
ing in  the  crevices  or  joints  between  the  stones,  these  rock 
growths  sometimes  prize  off  great  blocks.  In  other  cases 
the  weight  of  the  pendent  stalactite  drags  the  ill-supported 
masses  of  the  roof  to  the  floor.  In  this  way  a  gallery  origi- 
nally a  hundred  feet  below  the  surface  may  work  its  way 
upward  to  the  light  of  day.  The  entrance  by  which  the 
Mammoth  Cave  is  approached  appears  to  have  been  formed 
in  this  manner,  and  at  several  points  in  that  system  of 
caverns  the  effect  of  this  action  may  be  distinctly  observed. 

We  must  now  go  a  step  further  on  the  way  of  sub- 
terranean water,  and  trace  its  action  in  the  depths  below 
the  plane  of  ordinary  caves,  which,  as  we  have  noted,  do 
not  extend  below  the  level  of  the  main  streams  of  the 
cavern  district.  The  first  group  of  facts  to  be  attended 
to  is  that  exhibited  by  artesian  wells.  These  occur  where 
rocks  have  been  folded  down  into  a  basinlike  form.  It 
often  happens  that  in  such  a  basin  the  rocks  of  which  it 


*  It  is  reported  that  one  of  these  natural  bridges  of  Carter  Connty 
has  recently  fallen  down.  This  is  the  natural  end  of  these  features. 
As  before  remarked,  they  are  but  the  remnants  of  much  more  exten- 
sive roofs  which  the  processes  of  decay  have  brought  to  ruin. 


THE  WOllK  OF  UNDERGROUND  WATER.         259 

is  composed  are  some  of  them  porous,  and  others  imper- 
vious to  water,  and  that  the  porous  layers  outcrop  on  the 
high  margins  of  the  depression  and  have  water-tight  layers 
over  them.  These  conditions  can  be  well  represented  by 
supposing  that  we  have  two  saucers,  one  within  the  other, 
with  an  intervening  layer  of  sand  which  is  full  of  water. 
If  now  we  bore  an  opening  in  the  bottom  of  the  uppermost 
saucer,  we  readily  conceive  that  the  water  will  flow  up 
through  it.  In  Nature  we  often  find  these  basins  with 
the  equivalent  of  the  sandy  layer  in  the  model  just  de- 
scribed rising  hundreds  of  feet  above  the  valley,  so  that 
the  artesian  well,  so  named  from  the  village  of  Artois,  near 
Paris,  where  the  first  opening  of  this  nature  was  made,  may 
yield  a  stream  which  will  mount  upward,  especially  where 
piped,  to  a  great  height.  At  many  places  in  the  world 
it  is  possible  by  such  wells  to  obtain  a  large  supply  of  tol- 
erably pure  water,  but  in  general  it  is  found  to  contain  too 
large  a  supply  of  dissolved  mineral  matter  or  sulphuretted 
gases  to  be  satisfactory  for  domestic  purposes.  It  may  be 
well  to  note  the  fact  that  the  greater  part  of  the  so-called 
artesian  wells,  or  borings  which  deliver  water  to  a  height 
above  the  surface,  are  not  true  artesian  sources,  in  that 
they  do  not  send  up  the  water  by  the  action  of  gravitation, 
but  under  the  influence  of  gaseous  pressure. 

Where,  as  in  the  case  of  upturned  porous  beds,  the 
crevice  water  penetrates  far  below  the  earth's  surface  or 
the  open-air  streams  which  drain  the  water  away,  the  fluid 
acquires  a  considerable  increase  of  temperature,  on  the 
average  about  one  degree  Fahrenheit  for  each  eighty  feet 
of  descent.  It  may,  indeed,  become  so  heated  that  if  it 
were  at  the  earth's  surface  it  would  not  only  burst  into 
steam  with  a  vast  explosive  energy,  but  would  actually 
shine  in  the  manner  of  heated  solids.  As  the  temperature 
of  water  rises,  and  as  the  pressure  on  it  increases,  it  ac- 
quires a  solvent  power,  and  takes  in  rocky  matter  in  a 
measure  unapproached  at  the  earth's  surface.  At  the 
depth  of  ten  miles  water  beginning  as  inert  rain  would 


2G0 


OUTLINES  OF  THE  EARTH'S  HISTORY. 


acquire  the  properties  which  we  are  accustomed  to  associ- 
ate with  strong  acids.  Passing  downward  through  fissures 
or  porous  strata  in  the  manner  indicated  in  the  diagram, 
tlie  water  would  take  up,  by  virtue  of  its  heat  and  the 
gases  it  contained,  a  share  of  many  mineral  substances 
which  we  commonly  regard  as  insoluble.  Gold  and  even 
platinum — the  latter  a  material  which  resists  all  acids  at 
ordinary  temperatures — enters  into  the  solution.  If  now 
the  water  thus  charged  with  mineral  stores  finds  in  the 
depths  a  shorter  way  to  the  surface  than  that  which  it 
descended,  which  may  w^ell  happen  by  way  of  a  deep  rift 
in  the  rocks,  it  will  in  its  ascent  reverse  the  process  which 
it  followed  on  going  down.     It  will  deposit  the  several 

minerals  in  the  order  of 
their  solubilities — that  is, 
the  last  to  be  taken  in  will 
be  the  first  to  be  crystallized 
on  the  walls  of  the  fissure 
through  which  the  upflow  is 
taking  place.  The  result 
will  be  the  formation  of  a 
vein  belonging  to  the  vari- 
ety known  as  fissure  veins. 

A  vein  deposit  such  as 
we  are  considering  may, 
though  rarely,  be  composed 
of  a  single  mineral.  Most 
commonly  we  find  the  de- 
posit arranged  in  a  banded  form  in  the  manner  indicated 
in  the  figure  (see  diagram  14).  Sometimes  one  material 
will  abound  in  the  lower  portions  of  the  fissure  and  an- 
other in  its  higher  parts,  a  feature  which  is  accounted 
for  by  the  progressive  cooling  and  relinquishment  of  pres- 
sure to  which  the  water  is  subjected  on  it's  way  to  the 
surface.  With  each  decrement  of  those  properties  some 
particular  substance  goes  out  of  the  fluid,  which  may  in 
the  end  emerge  in  the  form  of  a  warm  or  hot  spring,  the 


h    b  a     c      d     d 

Fig.  14. — Diagram  of  vein.  The 
different  shadings  show  the 
variations  in  the  nature  of 
the  deposits. 


THE  WOllK  OF   UNDERGROUND  WATER.         2G1 

water  of  which  contains  but  little  mineral  matter.  Where, 
however,  the  temperature  is  high,  some  part  of  the  deposit, 
even  a  little  gold,  may  be  laid  down  just  about  the  spring 
in  the  deposits  known  as  sinter,  which  are  often  formed 
at  such  places. 

In  many  cases  the  ore  deposits  are  formed  not  only  in 
the  main  channel  of  the  fissure,  but  in  all  the  crevices  on 
either  side  of  that  way.  In  this  manner,  much  as  in  the 
case  of  the  growth  of  stalactitic  matter  between  the  blocks 
of  stone  in  the  roofs  of  a  cavern,  large  fragments  of  rock, 
known  as  "  horses,"  are  often  pushed  out  into  the  body 
of  the  vein.  In  some  instances  the  growth  of  the  vein 
appears  to  enlarge  the  fissure  or  place  of  the  deposit  as  the 
accumulation  goes  on,  the  process  being  analogous  to  that 
by  which  a  growing  root  widens  the  crevice  into  which 
it  has  penetrated.  In  other  instances  the  fissure  formed 
by  the  force  has  remained  wide  open,  or  at  most  has  been 
but  partly  filled  by  the  action  of  the  water. 

It  not  infrequently  happens  that  the  ascending  waters 
of  hot  springs  entering  limestones  have  excavated  ex- 
tensive caves  far  below  the  surface  of  the  earth,  these  cav- 
erns being  afterward  in  part  filled  by  the  ores  of  various 
metals.  We  can  readily  imagine  tliat  the  water  at  one  tem- 
perature would  excavate  the  cavern,  and  long  afterward, 
when  at  a  lower  heat,  they  might  proceed  to  fill  it  in.  At 
a  yet  later  stage,  when  the  surface  of  the  country  had 
worn  down  many  thousands  of  feet  below  the  original 
level,  the  mineral  stores  of  the  caverns  may  be  brought 
near  the  surface  of  the  earth.  Some  of  the  most  impor- 
tant metalliferous  deposits  of  the  Cordilleras  are  found 
in  this  group  of  hot-water  caverns.  These  caverns  are 
essentially  like  those  produced  by  cold  water,  with  the  ex- 
ception of  the  temperature  of  the  fluid  which  does  the 
work  and  the  opposite  direction  of  the  flow. 

In  following  crevice  water  which  is  free  to  obey  the 
impulses  of  gravitation  far  down  into  the  earth,  we  enter 
on  a  realm  where  the  rock  or  construction  water,  that 


262  OUTLINES  OF  THE  EARTH'S  HISTOHV. 

which  was  built  into  the  stone  at  the  time  of  its  formation, 
is  plentiful.  Where  these  two  groups  of  w^aters  come  in 
contact  an  admixture  occurs,  a  certain  portion  of  the  rock 
water  joining  that  in  the  crevices.  Near  the  surface  of 
the  ground  we  commonly  find  that  all  the  construction 
water  has  been  washed  out  by  this  action.  Yet  if  the  rocks 
be  compact,  or  if  they  have  layers  of  a  soft  and  clayey  na- 
ture, we  may  find  the  construction  water,  even  in  very 
old  deposits,  remaining  near  the  surface  of  the  ground. 
Thus  in  the  ancient  Siluriali  beds  of  the  Ohio  Valley  a 
boring  carried  a  hundred  feet  below  the  level  of  the  main 
rivers  commonly  discovers  water  which  is  clearly  that  laid 
down  in  the  crevices  of  the  material  at  the  time  when  the 
rocks  were  formed  in  the  sea.  In  all  cases  this  water  con- 
tains a  certain  amount  of  gases  derived  from  the  decomposi- 
tion of  various  substances,  but  principally  from  the  altera- 
tion of  iron  pyrite,  which  affords  sulphuretted  hydrogen. 
Thus  the  water  is  forced  to  the  surface  with  considerable 
energy,  and  the  well  is  often  named  artesian,  though  it 
flows  by  gas  pressure  on  the  principle  of  the  soda-water 
fountain,  and  not  by  gravity,  as  in  the  case  of  true  artesian 
wells. 

The  passage  between  the  work  done  by  the  deeply 
penetrating  surface  water  and  that  due  to  the  fluid  inti- 
mately blended  with  the  rock  built  into  the  mass  at  the 
time  of  its  formation  is  obscure.  We  are,  however,  quite 
sure  that  at  great  depths  beneath  the  earth  the  construc- 
tion water  acts  alone  not  only  in  making  veins,  but  in 
bringing  about  many  other  momentous  changes.  At  a 
great  depth  this  water  becomes  intensely  heated,  and  there- 
fore tends  to  move  in  any  direction  where  a  chance  fissure 
or  other  accident  may  lessen  the  pressure.  Creeping 
through  the  rocks,  and  moving  from  zones  of  one  tempera- 
ture to  another,  these  waters  bring  about  in  the  fine  inter- 
stices chemical  changes  which  lead  to  great  alterations 
in  the  constitution  of  the  rock  material.  It  is  probably 
in  part  to  these  slow  driftings  of  rock  water  that  beds 


THE  WORK  OF  UNDERGUOUND  WATER.         263 

originally  made  up  of  small,  shapeless  fragments,  such  as 
compose  clay  slates,  sandstones,  and  limestones,  may  in 
time  be  altered  into  crystalline  rocks,  where  there  is  no 
longer  a  trace  of  the  original  bits,  all  the  matter  having 
been  taken  to  pieces  by  the  process  of  dissolving,  and  re- 
formed in  the  regular  crystalline  order.  In  many  cases 
we  may  note  how  a  crystal  after  being  made  has  been  in 
part  dissolved  away  and  replaced  by  another  mineral.  In 
fact,  many  of  our  rocks  appear  to  have  been  again  and 
again  made  over  by  the  slow-drifting  waters,  each  particu- 
lar state  in  their  construction  being  due  to  some  peculiarity 
of  temperature  or  of  mineral  contents  which  the  fluid 
held.  These  metamorphic  phenomena,  though  important, 
are  obscure,  and  their  elucidation  demands  some  knowledge 
of  petrographic  science,  that  branch  of  geology  which  con- 
siders the  principles  of  rock  formation.  They  will  there- 
fore not  be  further  considered  in  this  work. 

Volcanoes. 

Of  old  it  was  believed  that  volcanoes  represented  the 
outpouring  of  fluid  rock  which  came  forth  from  the  cen- 
tral realm  of  the  earth,  a  region  which  was  supposed  still 
to  retain  the  liquid  state  through  which  the  whole  mass 
of  our  earth  has  doubtless  passed.  Recent  studies,  how- 
ever, have  brought  about  a  change  in  the  views  of  geolo- 
gists which  is  represented  by  the  fact  that  we  shall  treat 
volcanic  phenomena  in  connection  with  the  history  of 
rock  water. 

In  endeavouring  to  understand  the  phenomena  of  vol- 
canoes it  is  very  desirable  that  the  student  should  under- 
stand what  goes  on  in  a  normal  eruption.  The  writer  may, 
therefore,  be  warranted  in  describing  some  observations 
which  he  had  an  opportunity  to  make  at  an  eruption  of 
Vesuvius  in  1883,  when  it  was  possible  to  behold  far  more 
than  can  ordinarily  be  discerned  in  such  outbreaks — in 
fact,  the  opportunity  of  a  like  nature  has  probably  not 
18 


2t)4  OUTLINES  OF  THE  EARTH'S  HISTORY. 

been  enjoyed  by  any  other  person  interested  in  volcanic 
action.  In  the  winter  of  1882-'83  Vesuvius  was  subjected 
to  a  succession  of  slight  outbreaks.  At  the  time  of  the 
observations  about  to  be  noted  the  crater  had  been  reduced 
to  a  cup  about  three  hundred  feet  in  diameter  and  about 
a  hundred  feet  deep.  The  vertical  shaft  at  the  bottom, 
through  which  the  outbursts  were  taking  place,  was  about 
a  hundred  feet  across.  Taking  advantage  of  a  heavy  gale 
from  the  northwest,  it  was  practicable,  notwithstanding 
the  explosions,  to  climb  to  the  edge  of  the  crater  wall. 
Looking  down  into  the  throat  of  the  volcano,  although  the 
pit  was  full  of  whirling  vapours  and  the  heat  was  so  great 
that  the  protection  of  a  mask  was  necessary,  it  was  possible 
to  see  something  of  what  was  going  on  at  the  moment  of 
an  explosion. 

The  pipe  of  the  volcano  was  full  of  white-hot  lava. 
Even  in  a  day  of  sunshine,  which  was  only  partly  obscured 
by  the  vapours  which  hung  about  the  opening,  the  heat 
of  the  lava  made  it  very  brilliant.  This  mass  of  fluid  rock 
was  in  continuous  motion,  swaying  violently  up  and  down 
the  tube.  From  four  to  six  times  a  minute,  at  the  moment 
of  its  upswaying,  it  would  burst  as  by  the  explosion  of  a 
gigantic  bubble.  The  upper  portion  of  the  mass  was  blown 
upward  in  fragments,  the  discharge  being  like  that  of  shot 
from  a  fowling  piece;  the  fragments,  varying  in  size  from 
small,  shotlike  bits  to  masses  larger  than  a  man's  head, 
were  shot  up  sometimes  to  the  height  of  fifteen  hundred 
feet  above  the  point  of  ejection.  The  wind,  blowing  at 
the  rate  of  about  forty  miles  an  hour,  drove  the  falling 
bits  of  rock  to  the  leeward,  so  that  there  was  no  consider- 
able danger  to  be  apprehended  from  them.  Some  seconds 
after  the  explosion  they  could  be  heard  rattling  down  on 
the  farther  slope  of  the  cone.  Observations  on  the  interval 
between  the  discharge  and  the  fall  of  the  fragments  made 
it  easy  to  compute  the  height  to  which  they  were  thrown. 

At  the  moment  when  the  lava  in  the  pipe  opened  for 
the  passage  of  the  vapour  which  created  the  explosion  the 


TPIE   WORK  OF  UNDERGROUND  WATER.         265 

movement,  though  performed  in  a  fraction  of  a  second, 
was  clearly  visible.  At  first  the  vapour  was  colourless;  a 
few  score  feet  up  it  began  to  assume  a  faint,  bluish  hue; 
yet  higher,  when  it  was  more  expanded,  the  tint  changed 
to  that  of  steam,  which  soon  became  of  the  ordinary  aspect, 
and  gathered  in  swift-revolving  clouds.  The  watery  nature 
of  the  vapour  was  perfectly  evident  by  its  odour.  Though 
commingled  with  sulphurous-acid  gas,  it  still  had  the  char- 
acteristic smell  of  steam.  For  a  half  hour  it  was  possible 
to  watch  the  successive  explosions,  and  even  to  make  rough 
sketches  of  the  scene.  Occasionally  the  explosions  would 
come  in  quick  succession,  so  that  the  lava  was  blown  out 
of  the  tube;  again,  the  pool  would  merely  sway  up  and 
down  in  a  manner  which  could  be  explained  only  by  sup- 
posing that  great  bubbles  of  vapour  were  working  their 
way  upward  toward  the  point  where  they  could  burst. 
Each  of  these  bubbles  probably  filled  a  large  part  of  the 
diameter  of  the  pipe.  In  general,  the  phenomena  recalled 
the  escape  of  the  jet  from  a  geyser,  or,  to  take  a  familiar 
instance,  that  of  steam  from  the  pipe  of  a  high-pressure 
engine.  When  the  heat  is  great,  steam  may  often  be  seen 
at  the  mouth  of  the  pipe  with  the  same  transparent  ap- 
pearance which  was  observed  in  the  throat  of  the  crater. 
In  the  cold  air  of  the  mountain  the  vapour  was  rapidly 
condensed,  giving  a  rainbow  hue  in  the  clouds  when  they 
were  viewed  at  the  right  angle.  The  observations  were 
interrupted  by  tlie  fact  that  the  wind  so  far  died  away  that 
large  balls  of  the  ejected  lava  began  to  fall  on  the  wind- 
ward side  of  the  cone.  These  fragments,  though  cooled 
and  blackened  on  their  outside  by  their  considerable  jour- 
ney up  and  down  through  the  air,  were  still  so  soft  that 
they  splashed  when  they  struck  the  surface  of  cinders. 

Watching  the  cone  from  a  distance,  one  could  note  that 
from  time  to  time  the  explosions,  increasing  in  frequency, 
finally  attained  a  point  where  the  action  appeared  to  be 
continuous.  The  transition  was  comparable  to  that  which 
we  may  observe  in  a  locomotive  which,  when  it  first  gets 


26 J  OUTLINES  OF  THE  EARTH'S  IIISTOIIY. 

under  way,  gives  forth  occasional  jets  of  steam,  but,  slowly 
gaining  speed,  finally  pours  forth  what  to  eye  and  ear 
alike  seem  to  be  a  continuous  outrush.  All  the  evidence 
that  we  have  concerning  volcanic  outbreaks  corroborates 
that  just  cited,  and  is  to  the  effect  that  the  essence  of  the 
action  consists  in  the  outbreak  of  water  vapour  at  a  high 
temperature,  and  therefore  endowed  with  very  great  ex- 
pansive force.  Along  with  this  steam  there  are  many  other 
gases,  which  always  appear  to  be  but  a  very  small  part  of 
the  whole  escape  of  a  vaporous  nature — in  fact,  the  vol- 
canic steam,  so  far  as  its  chemical  composition  has  been 
ascertained,  has  the  composition  which  we  should  expect 
to  find  in  rock  water  which  had  been  forced  out  from  the 
rock  by  the  tensions  that  high  temperature  creates. 

Because  of  its  conspicuous  nature,  the  lava  which  flows 
from  most  volcanoes,  or  is  blown  out  from  them  in  the 
form  of  finely  divided  ash,  is  commonly  regarded  as  the 
primary  feature  in  a  volcanic  outbreak.  Such  is  not  really 
the  case.  Volcanic  explosions  may  occur  with  very  little 
output  of  fluid  rock,  and  that  which  comes  forth  may 
consist  altogether  of  the  finely  divided  bits  of  rock  to 
which  we  give  the  name  of  ash.  In  fact,  in  all  very  power- 
ful explosions  we  may  expect  to  find  no  lava  flow,  but 
great  quantities  of  this  finely  divided  rock,  which  when 
it  started  from  the  depths  of  the  earth  was  in  a  fluid  state, 
but  was  blown  to  pieces  by  the  contained  vapour  as  it  ap- 
proached the  surface. 

If  the  student  is  so  fortunate  as  to  behold  a  flood  of 
lava  coming  forth  from  the  flanks  of  a  volcano,  he  will 
observe  that  even  at  the  very  points  of  issue,  where  the 
material  is  white-hot  and  appears  to  be  as  fluid  as  water, 
the  whole  surface  gives  forth  steam.  On  a  still  day,  viewed 
from  a  distance,  the  path  of  a  lava  flow  is  marked  by  a 
dense  cloud  of  this  vapour  which  comes  forth  from  it.  Even 
after  the  lava  has  cooled  so  that  it  is  safe  to  walk  upon  it, 
every  crevice  continues  to  pour  forth  steam.  Years  after 
the  flowing  has  ceased,  and  when  the  rock  surface  has 


THE  WORK  OP  UNBERGROUKB  WATER.    007 

become  cool  enough  for  the  growth  of  certain  plants  upon 
it,  these  crevices  still  yield  steam.  It  is  evident,  in  a  word, 
that  a  considerable  part  of  a  lava  mass,  even  after  it  escapes 
from  the  volcanic  pipes,  is  water  which  is  intimately  com- 
mingled with  the  rock,  probably  lying  between  the  very 
finest  grains  of  the  heated  substance.  Yet  this  lava  which 
has  come  forth  from  the  volcano  has  only  a  portion  of  the 
water  which  it  originally  contained;  a  large,  perhaps  the 
greater  part,  has  gone  forth  in  the  explosive  way  through 
the  crater.  It  is  reasonably  believed  that  the  fluidity  of 
lava  is  in  considerable  measure  due  to  the  water  which  it 
contains,  and  which  serves  to  give  the  mass  the  consistence 
of  paste,  the  partial  fluidity  of  flour  and  rock  grains  being 
alike  brought  about  in  the  same  manner. 

So  much  of  the  phenomena  of  volcanoes  as  has  been 
above  noted  is  intended  to  show  the  large  part  which  inter- 
stitial water  pla3^s  in  volcanic  action.  We  shall  now  turn 
our  attention  again  to  the  state  of  the  deeply  buried  rock 
water,  to  see  how  far  we  may  be  able  by  it  to  account  for 
these  strange  explosive  actions.  When  sediments  are  laid 
down  on  the  sea  floor  the  materials  consist  of  small,  irregu- 
larly shaped  fragments,  which  lie  tumbled  together  in  the 
manner  of  a  mass  of  bricks  which  have  been  shot  out  of  a 
cart.  Water  is  buried  in  the  plentiful  interspaces  between 
these  bits  of  stone;  as  before  remarked,  the  amount  of  this 
construction  water  varies.  In  general,  it  is  at  first  not  far 
from  one  tenth  part  of  the  materials.  Besides  the  fluid  con- 
tained in  the  distinct  spaces,  there  is  a  share  which  is  held 
as  combined  water  in  the  intimate  structure  of  the  crystals, 
if  such  there  be  in  the  mass.  When  this  water  is  built  into 
the  stone  it  has  the  ordinary  temperature  of  the  sea  bottom. 
As  the  depositing  actions  continue  to  work,  other  beds  are 
formed  on  the  top  of  that  which  we  are  considering,  and 
in  time  the  layer  may  be  buried  to  the  depth  of  many 
thousand  feet.  There  are  reasons  to  believe  that  on  the 
floors  of  the  oceans  this  burial  of  beds  containing  water 
may  have  brought  great  quantities  of  fluid  to  the  depth 


268         OUTLINES  OF  THE  EARTH'S  HISTORY. 

of  twenty  miles  or  more  below  the  outer  surface  of  the 
rocks. 

The  effect  of  deep  burial  is  to  increase  the  heat  of 
strata.    This  result  is  accomplished  in  two  different  ways. 


Fig.  15. — Flow  of  lava  invading  a  forest.  A  tree  in  the  distance  is 
not  completely  burned,  showing  that  the  molten  rock  had  lost 
much  of  its  original  heat. 


The  direct  effect  arising  from  the  imposition  of  weight, 
that  derived  from  the  mass  of  stratified  material,  is,  as  we 
know,  to  bring  about  a  dow^n-sinking  of  the  earth's  crust. 
In  the  measure  of  this  falling,  heat  is  engendered  pre- 


THE  WORK  OF  UNDERGROUND  WATER.    269 

cisely  as  it  is  by  the  falling  of  a  trip-hammer  on  the  anvil, 
with  which  action,  as  is  well  known,  we  may  heat  an  iron 
bar  to  a  high  temperature.  It  is  true  that  this  down-sink- 
ing of  the  surface  under  weight  is  in  part  due  to  the  com- 
pression of  the  rocks,  and  in  part  to  the  slipping  away  of 
the  soft  underpinning  of  more  or  less  fluid  rock.  Yet 
further  it  is  in  some  measure  brought  about  by  the  wrin- 
kling of  the  crust.  But  all  these  actions  result  in  the  con- 
version of  energy  of  position  into  heat,  and  so  far  serve 
to  raise  the  temperature  of  the  rocks  which  are  concerned 
in  the  movements.  By  far  the  largest  source  of  heat, 
however,  is  that  which  comes  forth  from  the  earth's  in- 
terior, and  which  was  stored  there  in  the  olden  day  when 
the  matter  forming  the  earth  gathered  into  the  mass  of 
our  sphere.  This,  which  we  may  term  the  original  heat, 
is  constantly  flowing  forth  into  space,  but  makes  its  way 
slowly,  because  of  the  non-conductive,  or,  as  we  may  phrase 
it,  the  "  blanketing  "  effect  of  the  outer  rock.  The  effect 
of  the  strata  is  the  same  as  that  exercised  by  the  non-con- 
ductive coatings  which  are  put  on  steam  boilers.  A  more 
familiar  comparison  may  be  had  from  the  blankets  used 
for  bedclothing.  If  on  top  of  the  first  blanket  we  put  a 
second,  we  keep  warmer  because  the  temperature  of  the 
lower  one  is  elevated  by  the  heat  from  our  body  which  is 
held  in.  In  the  crust  of  the  earth  each  layer  of  rock  re- 
sists the  outflow  of  heat,  and  each  addition  lifts  the  tem- 
perature of  all  the  layers  below. 

When  water-bearing  strata  have  been  buried  to  the 
depth  of  ten  miles,  the  temperature  of  the  mass  may  be 
expected  to  rise  to  somewhere  between  seven  hundred 
and  a  thousand  degrees  Fahrenheit.  If  the  depth  attained 
should  be  fifty  miles,  it  is  likely  that  the  temperature  will 
be  five  times  as  great.  At  such  a  heat  the  water  which  the 
rocks  contain  tends  in  a  very  vigorous  way  to  expand  and 
pass  into  the  state  of  vapour.  This  it  can  not  readily  do, 
because  of  its  close  imprisonment;  we  may  say,  however, 
that  the  tendency  toward  explosion  is  almost  as  great  as 


270  OUTLINES  OF  THE  EARTH'S  HISTORY. 

that  of  ignited  gunpowder.  Such  powder^  if  held  in  small 
spaces  in  a  mass  of  cast  steel,  could  be  fired  without  rend- 
ing the  metal.  The  gases  would  be  retained  in  a  highly 
compressed,  possibly  in  a  fluid  form.  If  now  it  happens 
that  any  of  the  strain  in  the  rocks  such  as  lead  to  the  pro- 
duction of  faults  produce  fissures  leading  from  the  surface 
into  this  zone  of  heated  water,  the  tendency  of  the  rocks 
containing  the  fluid,  impelled  by  its  expansion,  will  be 
to  move  with  great  energy  toward  the  point  of  relief  or 
lessened  pressure  which  the  crevice  affords.  Where  rocks 
are  in  any  way  softened,  pressure  alone  will  force  them  into 
a  cavity,  as  is  shown  by  the  fact  that  beds  of  tolerably  hard 
clay  stones  in  deep  coal  mines  may  be  forced  into  the  spaces 
by  the  pressure  of  the  rocks  which  overlie  them — in  fact, 
the  expense  of  cutting  out  these  in-creeping  rocks  is  in 
some  British  mines  a  serious  item  in  the  cost  of  the  product. 
The  expansion  of  the  water  contained  in  the  deep- 
lying  heated  rocks  probably  is  by  far  the  most  efficient 
agent  in  urging  them  toward  the  plane  of  escape  which 
the  fissure  affords.  When  the  motion  begins  it  pervades  all 
parts  of  the  rock  at  once,  so  that  an  actual  flow  is  induced. 
So  far  as  the  movement  is  due  to  the  superincumbent 
weight,  the  tendency  is  at  once  to  increase  the  temperature 
of  the  moving  mass.  The  result  is  that  it  may  be  urged 
into  the  fissure  perhaps  even  hotter  than  when  it  started 
from  the  original  bed  place.  In  proportion  as  the  rocky 
matter  wins  its  way  toward  the  surface,  the  pressure  upon 
it  diminishes,  and  the  contained  vapours  are  freer  to  ex- 
pand. Taking  on  the  vaporous  form,  the  bubbles  gather 
to  each  other,  and  when  they  appear  at  the  throat  of  the 
volcano  they  may,  if  the  explosions  be  infrequent,  assume 
the  character  above  noted  in  the  little  eruption  of  Vesu- 
vius. Where,  however,  the  lava  ascends  rapidly  through 
the  channel,  it  often  attains  the  open  air  with  so  much 
vapour  in  it,  and  this  intimately  mingled  with  the  mass, 
that  the  explosion  rends  the  materials  into  an  impalpably 
fine  powder,  which  may  float  in  the  air  for  months  before 


THE  WORK  OF  UNDERGROUND  WATER.    271 

it  falls  to  the  earth.  With  a  less  violent  movement  the 
vapour  bubbles  expand  in  the  lava,  but  do  not  rend  it 
apart,  thus  forming  the  porous,  spongy  rock  known  as 
pumice.  With  a  yet  slower  ascent  a  large  part  of  the 
steam  may  go  away,  so  that  we  may  have  a  flow  of  lava 
welling  forth  from  the  vent,  still  giving  forth  steam,  but 
with  a  vapour  whose  tension  is  so  lowered  that  the  matter 
is  not  blown  apart,  though  it  may  boil  violently  for  a  time 
after  it  escapes  into  the  air. 

Although  the  foregoing  relatively  simple  explanation 
of  volcanic  action  can  not  be  said  as  yet  to  be  generally 
accepted  by  geologists,  the  reasons  are  sufficient  which 
lead  us  to  believe  that  it  accounts  for  the  main  features 
which  we  observe  in  this  class  of  explosions — in  other 
words,  it  is  a  good  working  hypothesis.  We  shall  now  pro- 
ceed in  the  manner  which  should  be  followed  in  all  natu- 
ral inquiry  to  see  if  the  facts  shown  in  the  distribution  of 
volcanoes  in  space  and  time  confirm  or  deny  the  view. 

The  most  noteworthy  feature  in  the  distribution  of  vol- 
canoes is  that,  at  the  present  time  at  least,  all  active  vents 
are  limited  to  the  sea  floors  or  to  the  shore  lands  within 
the  narrow  range  of  three  hundred  miles  from  the  coast. 
Wherever  we  find  a  coast  line  destitute  of  volcanoes,  as  is 
the  case  with  the  eastern  coast  of  North  and  South  America, 
it  appears  that  the  shore  has  recently  been  carried  into 
the  land  for  a  considerable  distance — in  other  words,  old 
coast  lines  are  normally  volcanic;  that  is,  here  and  there 
have  vents  of  this  nature.  Thus  the  North  Atlantic, 
the  coasts  of  which  appear  to  have  gone  inland  for  a  great 
distance  in  geologically  recent  times,  is  non-volcanic;  while 
the  Pacific  coast,  which  for  a  long  time  has  remained  in 
its  present  position,  has  a  singularly  continuous  line  of 
craters  near  the  shore  extending  from  Alaska  to  Tierra 
del  Fuego.  So  uninterrupted  is  this  line  of  volcanoes 
that  if  they  were  all  in  eruption  it  would  very  likely  be 
possible  to  journey  down  the  coast  without  ever  being  out 
of  sight  of  the  columns  of  vapour  which  they  would  send 


272  OUTLINES  OP  THE  EARTH'S  HISTORY. 

forth.  On  the  floor  of  the  sea  volcanic  peaks  appear  to  be 
very  widely  distributed;  only  a  few  of  them — those  which 
attain  the  surface  of  the  water — are  really  known,  but 
soundings  show  long  lines  of  elevations  which  doubtless 
represent  cones  distributed  along  fault  lines,  none  of  the 
peaks  of  sufficient  height  to  break  the  surface  of  the  sea.  It 
is  likely,  indeed,  that  for  one  marine  volcano  which  appears 
as  an  island  there  are  scores  which  do  not  attain  the  sur- 
face. Volcanic  islands  exist  and  generally  abound  in  the 
ocean  and  greater  seas;  every  now  and  then  we  observe  a 
new  one  forming  as  a  small  island,  which  is  apt  to  be 
washed  away  by  the  sea  shortly  after  the  eruption  ceases, 
the  disappearance  being  speedy,  for  the  reason  that  the 
volcanic  ashes  of  which  these  cones  are  composed  drift 
away  like  snow  before  the  movement  of  the  waves. 

If  the  waters  of  the  ocean  and  seas  were  drained  away 
so  that  we  could  inspect  the  portion  of  the  earth's  surface 
which  they  cover  as  readily  as  we  do  the  dry  lands,  the 
most  conspicuous  feature  would  be  the  innumerable  vol- 
canic eminences  which  lie  hidden  in  these  watery  realms. 
Wherever  the  observer  passed  from  the  centres  of  the  pres- 
ent lands  he  would  note  within  the  limits  of  those  fields 
only  mountains,  much  modified  by  river  action;  hills  which 
the  rivers  had  left  in  scarfing  away  the  strata;  and  dales 
which  had  been  carved  out  by  the  flowing  waters.  N"ear 
the  shore  lines  of  the  vanished  seas  he  would  begin  to  find 
mountains,  hills,  and  vales  occasionally  commingled  with 
volcanic  peaks,  those  structures  built  from  the  materials 
ejected  from  the  vents.  Passing  the  coast  line  to  the  sea- 
ward, the  hills  and  dales  would  quickly  disappear,  and 
before  long  the  mountains  would  vanish  from  his  way,  and 
he  would  gradually  enter  on  a  region  of  vast  rolling  plains 
beset  by  volcanic  peaks,  generally  accumulated  in  long 
ranges,  somewhat  after  the  manner  of  mountains,  but  dif- 
fering from  those  elevations  not  only  in  origin  but  in 
aspect,  the  volcanic  set  of  peaks  being  altogether  made  up 
of  conical,  cup-topped  elevations. 


THE  WORK  OF  UNDERGROUND  WATER.    273 

A  little  consideration  will  show  us  that  the  fact  of  vol- 
canoes being  in  the  limit  to  the  sea  floors  and  to  a  narrow 
fringe  of  shore  next  certain  ocean  borders  is  reconcilable 
with  the  view  as  to  their  formation  which  we  have  adopted. 
We  have  already  noted  the  fact  that  the  continents  are  old, 
which  implies  that  the  parts  of  the  earth  which  they 
occupy  have  long  been  the  seats  of  tolerably  continuous 
erosion.  Now  and  then  they  have  swung  down  partly  be- 
neath the  sea,  and  during  their  submersion  they  received 
a  share  of  sediments.  But,  on  the  whole,  all  parts  of  the 
lands  except  strips  next  the  coast  may  be  reckoned  as  hav- 
ing been  subjected  to  an  excess  of  wearing  action  far  ex- 
ceeding the  depositional  work.  Therefore,  as  we  readily  see, 
underneath  such  land  areas  there  has  been  no  blanketing 
process  going  on  which  has  served  to  increase  the  heat  in 
the  deep  underlying  rocks.  On  the  contrary,  it  would  be 
easy  to  show,  and  the  reader  may  see  it  himself,  that  the 
progressive  cooling  of  the  earth  has  probably  brought 
about  a  lowering  of  the  temperature  in  all  the  section  from 
the  surface  to  very  great  depths,  so  that  not  only  is  the 
rock  water  unaffected  by  increase  of  heat,  but  may  be 
actually  losing  temperature.  In  other  w^ords,  the  condi- 
tions which  we  assume  bring  about  volcanic  action  do  not 
exist  beneath  the  old  land. 

Beneath  the  seas,  except  in  their  very  greatest  depths, 
and  perhaps  even  there,  the  process  of  forming  strata  is 
continually  going  on.  Next  the  shores,  sometimes  for  a 
hundred  or  two  miles  away  to  seaward,  the  principal  con- 
tribution may  be  the  sediment  worn  from  the  lands  by  the 
waves  and  the  rivers.  Farther  away  it  is  to  a  large  extent 
made  up  of  the  remains  of  animals  and  plants,  which  when 
dying  give  their  skeletons  to  form  the  strata.  Much  of 
the  materials  laid  down — perhaps  in  all  more  than  half — 
consist  of  volcanic  dust,  ashes,  and  pumice,  which  drifts 
very  long  times  before  it  finds  its  way  to  the  bottom.  We 
have  as  yet  no  data  of  a  precise  kind  for  determining  the 
average  rate  of  accumulation  of  sediments  upon  the  sea 


2U          OUTLINES   OP  THE  EARTH'S  HISTORY. 

floor,  but  from  what  is  known  of  the  wearing  of  the  lands, 
and  the  amount  of  volcanic  waste  which  finds  its  way  to 
the  seas,  it  is  probably  not  less  than  about  a  foot  in  ten 
thousand  years;  it  is  most  likely,  indeed,  much  to  exceed 
this  amount.  From  data  afforded  by  the  eruptions  in 
Java  and  in  other  fields  where  the  quantity  of  volcanic 
dust  contributed  to  the  seas  can  be  estimated,  the  writer 
is  disposed  to  believe  that  the  average  rate  of  sedimenta- 
tion on  the  sea  floors  is  twice  as  great  as  the  estimate  above 
given. 

Accumulating  at  the  average  rate  of  one  foot  in  ten 
thousand  years,  it  would  require  a  million  years  to  produce 
a  hundred  feet  of  sediments;  a  hundred  million  to  form 
ten  thousand  feet,  and  five  hundred  million  to  create  the 
thickness  of  about  ten  miles  of  bed.  At  the  rate  of  two 
feet  in  ten  thousand  years,  the  thickness  accumulated 
would  be  about  twenty  miles.  When  we  come  to  consider 
the  duration  of  the  earth's  geologic  history,  w^e  shall  find 
reasons  for  believing  that  the  formation  of  sediment  may 
have  continued  for  as  much  as  five  hundred  million  years. 

The  foregoing  inquiries  concerning  the  origin  of  vol- 
canoes show  that  at  the  present  time  they  are  clearly  con- 
nected with  some  process  which  goes  on  beneath  the  sea. 
An  extension  of  the  inquiry  indicates  that  this  relation  has 
existed  in  earlier  geological  times;  for,  although  the  liv- 
ing volcanoes  are  limited  to  places  within  three  hundred 
miles  of  the  sea,  we  find  lava  flows,  ashes,  and  other  vol- 
canic accumulations  far  in  the  interior  of  the  continents, 
though  the  energy  which  brought  them  forth  to  the  earth's 
surface  has  ceased  to  operate  in  those  parts  of  the  land. 
In  these  cases  of  continental  volcanoes  it  generally,  if  not 
always,  appears  that  the  cessation  of  the  activity  attended 
the  removal  of  the  shore  line  of  the  ocean  or  the  disappear- 
ance of  great  inland  seas.  Thus  the  volcanoes  of  the  Yel- 
lowstone district  may  have  owed  their  activity  to  the  im- 
mense deposits  of  sediment  which  were  formed  in  the 
vast  fresh-water  lakes  which  during  the  later  Cretaceous 


THE  WOKK  OF  UNDERGROUND  WATER.    275 

and  early  Tertiary  times  stretched  along  the  eastern  face 
of  the  Rocky  Mountains,  forming  a  Mediterranean  Sea  in 
North  America  comparable  to  that  which  borders  southern 
Europe.  It  thus  appears  that  the  arrangement  of  volcanoes 
with  reference  to  sea  basins  has  held  for  a  considerable 
period  in  the  past.  Still  further,  when  we  look  backward 
through  the  successive  formations  of  the  earth's  crust  we 
find  here  and  there  evidences  in  old  lava  flows,  in  volcanic 
ashes,  and  sometimes  in  the  ruins  of  ancient  cones  which 
have  been  buried  in  the  strata,  that  igneous  activity  such 
as  is  now  displayed  in  our  volcanoes  has  been,  since  the 
earliest  days  of  which  we  have  any  record,  a  characteristic 
feature  of  the  earth.  There  is  no  reason  to  suppose  that 
this  action  has  in  the  past  been  any  greater  or  any  less 
than  in  modern  days.  All  these  facts  point  to  the  conclu- 
sion that  volcanic  action  is  due  to  the  escape  of  rock  water 
which  has  been  heated  to  high  temperatures,  and  which 
drives  along  with  it  as  it  journeys  toward  a  crevice  the 
rock  in  which  it  has  been  confined. 

We  will  now  notice  some  other  explanations  of  volcanic 
action  which  have  obtained  a  certain  credence.  First,  we 
may  note  the  view  that  these  ejections  from  craters  are 
forced  out  from  a  supposed  liquid  interior  of  the  earth. 
One  of  the  difficulties  of  this  view  is  that  we  do  not  know 
that  the  earth's  central  parts  are  fluid — in  fact,  many  con- 
siderations indicate  that  such  is  not  the  case.  Next,  we 
observe  that  we  not  infrequently  find  two  craters,  each  con- 
taining fluid  lava,  with  the  fluid  standing  at  differences 
of  height  of  several  thousand  feet,  although  the  cones  are 
situated  very  near  each  other.  If  these  lavas  came  from 
a  common  internal  reservoir,  the  principles  which  con- 
trol the  action  of  fluids  would  cause  the  lavas  to  be  at  the 
same  elevation.  Moreover,  this  view  does  not  provide  any 
explanation  of  the  fact  that  volcanoes  are  in  some  way 
connected  with  actions  which  go  on  on  the  floors  of  great 
water  basins.  There  is  every  reason  to  believe  that  the 
fractures  in  the  rocks  under  the  land  are  as  numerous  and 


276  OUTLINES  OF   THE  EARTH'S  HISTORY. 

deep-going  as  those  beneath  the  sea.  If  it  were  a  mere 
question  of  access  to  a  fluid  interior,  volcanoes  should  be 
equally  distributed  on  land  and  sea  floors.  Last  of  all,  this 
explanation  in  no  wise  accounts  for  the  intermixture  of 
water  with  the  fluid  rock.  We  can  not  well  believe  that 
water  could  have  formed  a  part  of  the  deeper  earth  in  the 
old  days  of  original  igneous  fusion.  In  that  time  the 
water  must  have  been  all  above  the  earth  in  the  vaporous 
«tate. 

Another  supposition  somewhat  akin  to  that  mentioned 
is  that  the  water  of  the  seas  finds  its  way  down  through 
crevices  beneath  the  floors  of  the  ocean,  and,  there  com- 
ing in  contact  with  an  internal  molten  mass,  is  converted 
into  steam,  which,  along  with  the  fluid  rock,  escapes  from 
the  volcanic  vent.  In  addition  to  the  objections  urged  to 
the  precding  view,  we  may  say  concerning  this  that  the  lava, 
if  it  came  forth  under  these  circumstances,  would  emerge 
by  the  short  way,  that  by  which  the  water  went  down,  and 
not  by  the  longer  road,  by  which  it  may  be  discharged  ten 
thousand  feet  or  more  above  the  level  of  the  sea. 

The  foregoing  general  account  of  volcanic  action 
should  properly  be  followed  by  some  account  of  what  takes 
place  in  characteristic  eruptions.  This  history  of  these 
matters  is  so  ample  that  it  would  require  the  space  of  a 
great  encyclopaedia  to  contain  them.  We  shall  therefore 
be  able  to  make  only  certain  selections  which  may  serve  to 
illustrate  the  more  important  facts. 

By  far  the  best-known  volcanic  cone  is  that  of  Vesu- 
vius, which  has  been  subjected  to  tolerably  complete 
record  for  about  twenty-four  hundred  years.  About  500 
B.  c.  the  Greeks,  who  were  ever  on  the  search  for  places 
where  they  might  advantageously  plant  colonies,  settled 
on  the  island  of  Ischia,  which  forms  the  western  of  what, 
is  now  termed  the  Bay  of  Naples.  This  island  was  well 
placed  for  tillage  as  well  as  for  commerce,  but  the  enter- 
prising colonists  were  again  and  again  disturbed  by  vio- 
lent outbreaks  of  one  or  more  volcanoes  which  lie  in  the 


THE  WORK  OF  UNDERGROUND  WATER.         277 

interior  of  this  island;  at  one  time  it  appears  that  the 
people  were  driven  away  by  these  explosions. 

In  these  pre-Christian  days  Vesuvius,  then  known  as 
Monte  Somma,  was  not  known  to  be  a  volcano,  it  never 
having  shown  any  trace  of  eruption.  It  appeared  as  a 
regularly  shaped  mountain,  somewhat  over  two  thousand 
feet  high,  with  a  central  depression  about  three  miles  in 
diameter  at  the  top,  and  perhaps  two  miles  over  at  the 
bottom,  which  was  plainlike  in  form,  with  some  lakes  of 
bitter  water  in  the  centre.  The  most  we  know  of  this  cen- 
tral cavity  is  connected  with  the  insurrection  of  the  slaves 
led  by  Spartacus,  the  army  of  the  revolters  having  camped 
for  a  time  on  the  plain  encircled  by  the  crater  walls.  The 
outer  slopes  of  the  mountain  afforded  then  a  remarkably 
fertile  soil;  some  traces,  indeed,  of  the  fertility  have  with- 
stood the  modern  eruptions  which  have  desolated  its  flanks. 
This  wonderful  Bay  of  Naples  became  the  seat  of  the  fair- 
est Roman  culture,  as  well  as  of  a  very  extended  commerce. 
Toward  the  close  of  the  first  century  of  our  era  the  region 
was  perhaps  richer,  more  beautifully  cultivated,  and  the 
seat  of  a  more  elaborate  luxury  than  any  part  of  the  shore 
line  of  Europe  at  the  present  day.  At  the  foot  of  the 
mountain,  on  the  eastern  border  of  the  bay,  the  city  of 
Pompeii,  with  a  population  of  about  fifty  thousand  souls, 
was  a  considerable  port,  with  an  extensive  commerce,  par- 
ticularly with  Egypt.  The  charming  town  was  also  a  place 
of  great  resort  for  rich  Egyptians  who  cared  to  dwell  in 
Europe.  On  the  flanks  of  the  mountain  there  was  at  least 
one  large  town,  Ilerculaneum,  which  appears  to  have  been 
an  association  of  rich  men's  residences.  On  the  eastern 
side  of  the  bay,  at  a  point  now  known  as  Baiae,  the  Roman 
Government  had  a  naval  station,  which  in  the  year  79  was 
under  the  command  of  the  celebrated  Pliny,  a  most  volumi- 
nous though  unscientific  writer  on  matters  of  natural 
history.  With  him  in  that  year  there  was  his  nephew, 
commonly  known  as  the  younger  Pliny,  then  a  student  of 
eighteen  years,  but  afterward  himself  an  author.     These 


278  OUTLINES  OF  THE  EARTH'S  HISTORY. 

facts  are  stated  in  some  detail,  for  they  are  all  involved  in 
the  great  tragedy  which  we  are  now  to  describe. 

For  many  years  there  had  been  no  eruption  about  the 
Bay  of  Naples.  The  volcanoes  on  Ischia  had  been  still  for 
a  century  or  more,  and  the  various  circular  openings  on 
the  mainland  had  been  so  far  quiet  that  they  were  not 
recognised  as  volcanoes.  Even  the  inquisitive  Pliny,  with 
his  great  learning,  was  so  little  of  a  geologist  that  he  did 
not  know  the  signs  which  indicate  the  seat  of  volcanic 
action,  though  they  are  among  the  most  conspicuous  fea- 
tures which  can  meet  the  eye.  The  Greeks  would  doubt- 
less have  recognised  the  meaning  of  these  physical  signs. 
In  the  year  63  the  shores  of  the  Bay  of  Naples  were 
subjected  to  a  distinctive  earthquake.  Others  less 
severe  followed  in  subsequent  years.  In  an  early 
morning  in  the  year  79,  a  servant  aroused  the  elder 
Pliny  at  Baise  with  the  news  that  there  was  a  won- 
derful cloud  rising  from  Monte  Somma.  The  younger 
Pliny  states  that  in  form  it  was  like  a  pine  tree,  the 
common  species  in  Italy  having  a  long  trunk  with  a 
crown  of  foliage  on  its  summit,  shaped  like  an  umbrella. 
This  crown  of  the  column  grew  until  it  spread  over  the 
whole  landscape,  darkening  the  field  of  view.  Shortly 
after,  a  despatch  boat  brought  a  message  to  the  admiral, 
who  at  once  set  forth  for  the  seat  of  the  disturbance. 
He  invited  his  nephew  to  accompany  him,  but  the  pru- 
dent young  man  relates  in  his  letters  to  Tacitus,  from 
whom  we  know  the  little  concerning  the  eruption  which 
has  come  down  to  us,  that  he  preferred  to  do  some  read- 
ing which  he  had  to  attend  to.  His  uncle,  however,  went 
straight  forward,  intending  to  land  at  some  point  on 
the  shore  at  the  foot  of  the  cone.  He  found  the  sea, 
however,  so  high  that  a  landing  was  impossible;  more- 
over, the  fall  of  rock  fragments  menaced  the  ship.  He 
therefore  cruised  along  the  shore  for  some  distance,  land- 
ing at  a  station  probably  near  the  present  village  of  Cas- 
tellamare.    At  this  point  the  fall  of  ashes  and  pumice  was 


THE  WORK  OF  UNDERGROUND  WATER.    279 

very  great,  but  the  sturdy  old  Eoman  had  his  dinner  and 
slept  after  it.  There  is  testimony  that  he  snored  loudly, 
and  was  aroused  only  when  his  servants  began  to  fear  that 
the  fall  of  ashes  and  stones  would  block  the  way  out  of  his 
bedchamber.  When  he  came  forth  with  his  attendants, 
their  heads  protected  by  planks  resting  on  pillows,  he  set 
out  toward  Pompeii,  which  was  probably  the  place  where 
he  sought  to  land.  After  going  some  distance,  the  brave 
man  fell  dead,  probably  from  heart  disease;  it  is  said  that 
he  was  at  the  time  exceedingly  asthmatic.  No  sooner  were 
his  servants  satisfied  that  the  life  had  passed  from  his  body 
than  they  fled.  The  remains  were  recovered  after  the  erup- 
tion had  ceased.  The  younger  Pliny  further  relates  that 
after  his  uncle  left,  the  cloud  from  the  mountain  became 
so  dense  that  in  midday  the  darkness  was  that  of  midnight, 
and  the  earthquake  shocks  were  so  violent  that  wagons 
brought  to  the  courtyard  of  the  dwelling  to  bear  the  mem- 
bers of  the  household  away  were  rolled  this  way  and  that 
by  the  quakings  of  the  earth. 

Save  for  the  above-mentioned  few  and  unimportant 
details  concerning  the  eruption,  we  have  no  other  contem- 
poraneous account.  We  have,  indeed,  no  more  extended 
story  until  Dion  Cassius,  writing  long  after  the  event,  tells 
us  that  Herculaneum  and  Pompeii  were  overwhelmed;  but 
he  mixes  his  story  with  fantastic  legends  concerning  the 
appearance  of  gods  and  demons,  as  is  his  fashion  in  his 
so-called  history.  Of  all  the  Roman  writers,  he  is  perhaps 
the  most  untrustworthy.  Fortunately,  however,  we  have  in 
the  deposits  of  ashes  which  were  thrown  out  at  the  time 
of  this  great  eruption  some  basis  for  interpreting  the 
events  which  took  place.  It  is  evident  that  for  many 
hours  the  Yesuvian  crater,  which  had  been  dormant  for  at 
least  five  hundred  years,  blew  out  with  exceeding  fury. 
It  poured  forth  no  lava  streams;  the  energy  of  the  uprush- 
ing  vapours  was  too  great  for  that.  The  molten  rock  in 
their  path  was  blown  into  fine  bits,  and  all  the  hard  ma- 
terial cast  forth  as  free  dust.  In  the  course  of  the  erup- 
19 


280  OUTLINES  OF  THE  EARTH'S  HISTORY. 

tion,  which  probably  did  not  endure  more  than  two  days, 
possibly  not  more  than  twenty-four  hours,  ash  enough 
was  poured  forth  to  form  a  thick  layer  which  spread  far 
over  the  neighbouring  area  of  land  and  sea  floor.  It  cov- 
ered the  cities  of  Herculaneum  and  Pompeii  to  a  depth  of 
more  than  twenty  feet,  and  over  a  circle  having  a  diameter 
of  twenty  miles  the  average  thickness  may  have  been  some- 
thing like  this  amount.  So  deep  was  it  that,  although 
almost  all  the  people  of  these  towns  survived,  it  did  not 
seem  to  them  worth  while  to  undertake  to  excavate  their 
dwelling  places.  At  Pompeii  the  covering  did  not  overtop 
the  higher  of  the  low  houses.  An  amount  of  labour  which 
may  be  estimated  at  not  over  one  thirtieth  of  the  value, 
or  at  least  the  cost  which  had  been  incurred  in  building 
the  city,  would  have  restored  it  to  a  perfectly  inhabitable 
state.  The  fact  that  it  was  utterly  abandoned  probably 
indicates  a  certain  superstitious  view  in  connection  with 
the  eruption. 

The  fact  that  the  people  had  time  to  flee  from  Hercu- 
laneum and  Pompeii,  bearing  with  them  their  more  valu- 
able effects,  is  proved  by  the  excavations  at  these  places 
which  have  been  made  in  modern  times.  The  larger  part 
of  Pompeii  and  a  considerable  portion  of  Herculaneum 
have  been  thus  explored;  only  rarely  have  human  remains 
been  found.  Here  and  there,  particularly  in  the  cellars, 
the  labourers  engaged  in  the  work  of  disinterring  the 
cities  note  that  their  picks  enter  a  cavity;  examining  the 
space,  they  find  they  have  discovered  the  remains  of  a 
human  skeleton.  It  has  recently  been  learned  that  by 
pouring  soft  plaster  of  Paris  into  these  openings  a  mould 
may  be  obtained  which  gives  in  a  surprisingly  perfect  man- 
ner the  original  form  of  the  body.*  The  explanation  of 
this  mould  is  as  follows:  Along  with  the  fall  of  cinders 
in  an  eruption  there  is  always  a  great  descent  of  rain, 
arising  from  the  condensation  of  the  steam  which  pours 
forth  from  the  volcano.  This  water,  mingling  with  the 
ashes,  forms  a  pasty  mud,  which  often  flows  in  vast  streams^ 


THE  WORK  OF  UNDERGROUND  WATER.         2  SI 

and  is  sometimes  known  as  mud  lava.  This  material  has 
the  qualities  of  cement — that  is,  it  shortly  "  sets  "  in  a 
manner  comparable  to  plaster  of  Paris  or  ordinary  mortar. 
During  the  eruption  of  79  this  mud  penetrated  all  the  low 
places  in  Pompeii,  covering  the  bodies  of  the  people,  who 
were  suffocated  by  the  fumes  of  the  volcanic  emanations. 
We  know  that  these  people  were  not  drowned  by  the  in- 
undation; their  attitudes  show  that  they  were  dead  before 
the  flowing  matter  penetrated  to  where  they  lay. 

It  happened  that  Pompeii  lay  beyond  the  influence  of 
the  subsequent  great  eruptions  of  Vesuvius,  so  that  it 
afterward  received  only  slight  ash  showers.  Herculaneum, 
on  the  other  hand,  has  century  by  century  been  more  and 
more  deeply  buried  until  at  the  present  time  it  is  covered 
by  many  sheets  of  lava.  This  is  particularly  to  be  regretted, 
for  the  reason  that,  while  Pompeii  was  a  seaport  town  of 
no  great  wealth  or  culture,  Herculaneum  was  the  resi- 
dence place  of  the  gentry,  people  who  possessed  libraries, 
the  records  of  which  can  be  in  many  cases  deciphered, 
and  from  which  we  might  hope  to  obtain  some  of  the  lost 
treasures  of  antiquity.  The  papyrus  rolls  on  which  the 
books  of  that  day  were  written,  though  charred  by  heat 
and  time,  are  still  interpretable. 

After  the  great  explosion  of  79,  Vesuvius  sank  again 
into  repose.  It  was  not  until  1056  that  vigorous  erup- 
tions again  began.  From  time  to  time  slight  explosions 
occurred,  none  of  which  yielded  lava  flows;  it  was  not 
until  the  date  last  mentioned  that  this  accompaniment  of 
the  eruption  began  to  appear.  In  1636,  after  a  repose 
of  nearly  a  century  and  a  half,  there  came  a  very  great 
outbreak,  which  desolated  a  wide  extent  of  country  on  the 
northwestern  side  of  the  cone.  At  this  stage  in  the  his- 
tory of  the  crater  the  volcanic  flow  began  to  attain  the 
sea.  Washing  over  the  edge  of  the  old  original  crater  of 
Monte  Somma,  and  thus  lowering  its  elevation,  these 
streams  devastated,  during  the  eruption  just  mentioned 
and  in  various  other  outbreaks^  a  wide  field  of  cultivated 


282  OUTLINES   OF  THE  EARTH'S  HISTORY. 

land,  overwhelming  many  villages.  The  last  considerable 
eruption  which  yielded  large  quantities  of  lava  was  that 
of  1872,  which  sent  its  tide  for  a  distance  of  about  six 
miles. 

Since  1636  the  eruptions  of  Vesuvius  have  steadily  in- 
creased in  frequency,  and,  on  the  whole,  diminished  in 
violence.  In  the  early  years  of  its  history  the  great  out- 
breaks were  usually  separated  by  intervals  of  a  century 
or  more,  and  were  of  such  energy  that  the  lava  was  mostly 
blown  to  dust,  forming  clouds  so  vast  that  on  two  occa- 
sions at  least  they  caused  a  midnight  darkness  at  Con- 
stantinople, nearly  twelve  hundred  miles  away.  This  is 
as  if  a  volcano  at  Chicago  should  completely  hide  the  sun 
in  the  city  of  Boston.  In  the  present  state  of  Vesuvius, 
the  cone  may  be  said  to  be  in  slight,  almost  continuous 
eruption.  The  old  central  valley  which  existed  before 
the  eruption  of  79,  and  continued  to  be  distinct  for  long 
after  that  time,  has  been  filled  up  by  a  smaller  cone,  bear- 
ing a  relatively  tiny  crater  of  vent,  the  original  Avail  being 
visible  only  on  the  eastern  and  northern  parts  of  its  circuit, 
and  here  only  with  much  diminished  height.  On  the  west- 
ern face  the  slope  from  the  base  of  the  mountain  to  the 
summit  of  the  new  cone  is  almost  continuous,  though  the 
trained  eye  can  trace  the  outline  of  Monte  Somma — its  po- 
sition in  a  kind  of  bench,  which  is  traceable  on  that  side 
of  the  long  slope  leading  from  the  summit  of  the  new  cone 
to  the  sea.  The  fact  that  the  lavas  of  Vesuvius  have 
broken  out  on  the  southwestern  side,  while  the  old  wall 
of  the  cone  has  remained  unbroken  on  the  eastern  versant, 
has  a  curious  explanation.  The  prevailing  w^ind  of  Naples 
is  from  the  southwest,  being  the  strong  counter  trades 
which  belong  in  that  latitude.  In  the  old  days  when  the 
Monte  Somma  cone  was  constructed  these  winds  caused 
the  larger  part  of  the  ashes  to  fall  on  the  leeward  side  of 
the  cone,  thus  forming  a  thicker  and  higher  wall  around 
that  part  of  the  crater. 

From  the  nature  of  the  recent  eruptions  of  Vesuvius  it 


THE  WORK  OF  UNDERGROUND  WATER. 


2S3 


appears  likely  that  the  mountain  is  about  to  enter  on  a 
second  period  of  inaction.  The  pipes  leading  through  the 
new  cone  are  small,  and  the  mass  of  this  elevation  consti- 
tutes a  great  plug,  closing  the  old  crater  mouth.  To  give 
vent  to  a  large  discharge  of  steam,  the  whole  of  this  great 


63    A.  D. 


1868. 

Fig.  16. — Diagrammatic  sections  through  Mount  Vesuvius,  showing 
changes  in  the  form  of  the  cone.    (From  Phillips.) 

mass,  having  a  depth  of  nearly  two  thousand  feet,  would 
have  to  be  blown  away.  It  seems  most  likely  that  when  the 
occasion  for  such  a  discharge  comes,  the  vapours  of  the 
eruption  will  seek  a  vent  through  some  other  of  the  many 
volcanic  openings  which  lie  to  the  westward  of  this  giieat 
cone.  The  history  of  these  lesser  volcanoes  points  to  the 
conclusion  that  when  the  path  by  way  of  Vesuvius  is  ob- 
structed they  may  give  relief  to  the  steam  which  is  forcing 
its  course  to  the  surface.  Two  or  three  times  since  the 
eruption   of   Pliny,   during   periods   when   Vesuvius   liad 


284:  OUTLINES  OF  THE  EARTH'S  HISTORY. 

long  been  quiet,  outbreaks  have  taken  place  on  Ischia  or 
in  the  Phlaegrsen  Fields,  a  region  dotted  with  small  craters 
which  lies  to  the  west  of  Naples.  The  last  of  these  oc- 
curred in  1553,  and  led  to  the  formation  of  the  beautiful 
little  cone  known  as  Monte  Nuovo.  This  eruption  took 
place  near  the  town  of  Puzzuoli,  a  place  which  was  then 
the  seat  of  a  university,  the  people  of  which  have  left  us 
records  of  the  accident. 

The  outbreak  which  formed  Monte  Nuovo  was  slight 
but  very  characteristic.  It  occurred  in  and  beside  a  cir- 
cular pool  known  as  the  Lucrine  Lake,  itself  an  ancient 
crater.  At  the  beginning  of  the  disturbance  the  ground 
opened  in  ragged  cavities,  from  which  mud  and  ashes  and 
great  fragments  of  hard  rock  were  hurled  high  in  the  air, 
some  of  the  stones  ascending  to  a  height  of  several  thou- 
sand feet.  With  slight  intermissions  this  outbreak  con- 
tinued for  some  days,  resulting  in  the  formation  of  a  hill 
about  five  hundred  feet  high,  with  a  crater  in  its  top,  the 
bottom  of  which  lay  near  the  level  of  the  sea.  Although 
this  volcanic  elevation,  being  made  altogether  of  loose 
fragments,  is  rapidly  wearing  down,  while  the  crater  is 
filling  up,  it  remains  a  beautiful  object  in  the  landscape, 
and  is  also  noteworthy  for  the  fact  that  it  is  the  only  struc- 
ture of  this  nature  which  we  know  from  its  beginning. 
In  the  Phlaegraen  Field  there  are  a  number  of  other  craters 
of  small  size,  with  very  low  cones  about  them.  These  ap- 
pear to  have  been  the  product  of  brief,  slight  eruptions. 
That  known  as  the  Solfatara,  though  not  in  eruption  dur- 
ing the  historic  period,  is  interesting  for  the  fact  that  from 
the  crevices  of  the  rocks  about  it  there  comes  forth  a  con- 
tinued efflux  of  carbonic-acid  gas.  This  substance  prob- 
ably arises  from  the  effect  of  heat  contained  in  old  lavas 
which  are  in  contact  with  limestone  in  the  deep  under- 
earth.  We  know  such  limestones  are  covered  by  the  lavas 
-of  Vesuvius,  for  the  reason  that  numerous  blocks  of  the 
rock  are  thrown  out  during  eruptions,  and  are  often  found 
embedded  in  the  lava  streams.     It  is  an  interesting  fact 


THE  WORK  OF  UNDERGROUND  WATER.    285 

that  these  craters  of  the  Phlsegraen  Field,  lying  between 
the  seats  of  vigorous  eruption  on  Ischia  and  at  Vesuvius, 
have  never  been  in  vigorous  eruption.  Their  slight  out- 
breaks seem  to  indicate  that  they  have  no  permanent  con- 
nection with  the  sources  whence  those  stronger  vents  ob- 
tain their  supply  of  heated  steam. 

The  facts  disclosed  by  the  study  X)i  the  Yesuvian  system 
of  volcanoes  afford  the  geologist  a  basis  for  many  interest- 
ing conclusions. 

In  the  first  place,  he  notes  that  the  greater  part  of  the 
cones,  all  those  of  small  size,  are  made  up  of  finely  divided 
rock,  which  may  have  been  more  or  less  cemented  by  the 
processes  of  change  which  go  on  within  it.  It  is  thus  clear 
that  the  lava  flows  are  unessential — indeed,  we  may  say 
accidental — contributions  to  the  mass.  In  the  case  of 
Vesuvius  they  certainly  do  not  amount  to  as  much  as  one 
tenth  of  the  elevation  due  to  the  volcanic  action.  The 
share  of  the  lava  in  Vesuvius  is  probably  greater  than  the 
average,  for  during  the  last  six  centuries  this  vent  has 
been  remarkably  lavigerous.*  Observation  on  the  volcanoes 
of  other  districts  show  that  the  Vesuvian  group  is  in  this 
regard  not  peculiar.  Of  nearly  two  hundred  cones  which 
the  writer  has  examined,  not  more  than  one  tenth  disclose 
distinct  lavas. 

An  inspection  of  the  old  inner  wall  of  Monte  Somma 
in  that  portion  where  it  is  best  preserved,  on  the  north 
side  of  the  Atria  del  Cavallo,  or  Horse  Gulch — so  called 
for  the  reason  that  those  who  ascended  Vesuvius  were  ac- 
customed to  leave  their  saddle  animals  there — we  perceive 
that  the  body  of  the  old  cone  is  to  a  considerable  extent 
interlaced  with  dikes  or  fissures  which  have  been  filled 
with  molten  lava  that  has  cooled  in  its  place.  It  is  evi- 
dent that  during  the  throes  of  an  eruption,  when  the  lava 

*  I  venture  to  use  this  word  in  place  of  the  phrase  "  lava-yield- 
ing" for  the  reason  that  the  term  is  needed  in  the  description  of 
volcanoes. 


286  OUTLINES  OF  THE  EARTH'S  HISTORY. 

stands  high  in  the  crater,  these  rents  are  frequently  formed, 
to  be  filled  by  the  fluid  rock.  In  fact,  lava  discharges, 
though  they  may  afterward  course  for  long  distances  in  the 
open  air,  generally  break  their  way  underground  through 
the  cindery  cone,  and  first  are  disclosed  at  the  distance 
of  a  mile  or  more  from  the  inner  walls  of  the  crater.  Their 
path  is  probably  formed  by  riftings  in  the  compacted  ashes, 
such  as  we  trace  on  the  steep  sides  of  the  Atria  del  Cavallo, 
as  before  noted.  For  the  further  history  of  these  fissures, 
we  shall  have  to  refer  to  facts  which  are  better  exhibited 
in  the  cone  of  ^tna. 

The  amount  of  rock  matter  which  has  been  thrown 
forth  from  the  volcanoes  about  the  Bay  of  Naples  is  very 
great.  Only  a  portion  of  it  remains  in  the  region  around 
these  cones;  by  far  the  greater  part  has  been  washed  or 
blown  away.  After  each  considerable  eruption  a  wide 
field  is  coated  with  ashes,  so  that  the  tilled  grounds  appear 
as  if  entirely  sterilized;  but  in  a  short  time  the  matter  in 
good  part  disappears,  a  portion  of  it  decays  and  is  leached 
away,  and  the  most  of  the  remainder  washes  into  the  sea. 
Only  the  showers,  which  accumulate  a  deep  layer,  are  apt 
to  be  retained  on  the  surface  of  the  country.  A  great  deal 
of  this  powdered  rock  drifts  away  in  the  wind,  sometimes 
in  great  quantities,  as  in  those  cases  where  it  darkened  the 
sky  more  than  a  thousand  miles  from  the  cone.  Moreover, 
the  water  of  the  steam  which  brought  about  the  discharges 
and  the  other  gases  which  accompanied  the  vapour  have 
left  no  traces  of  their  presence,  except  in  the  deep  chan- 
nels which  the  rain  of  the  condensing  steam  have  formed 
on  the  hillsides.  Nevertheless,  after  all  these  subtractions 
are  made,  the  quantity  of  volcanic  matter  remaining  on  the 
surface  about  the  Bay  of  Naples  would,  if  evenly  dis- 
tributed, form  a  layer  several  hundred  feet  in  thickness — 
perhaps,  indeed,  a  thousand  feet  in  depth — over  the  terri- 
tory in  which  the  vents  occur.  All  this  matter  has  been 
taken  in  relatively  recent  times  from  the  depths  of  the 
earth.     The  surprising  fact  is  that  no  considerable  and. 


THE  WORK  OF  UNDERGROUND   WATER.         287 

indeed,  no  permanent  subsidence  of  the  surface  has  at- 
tended this  excavation.  We  can  not  believe  that  this 
withdrawal  of  material  from  the  under-earth  has  resulted 
in  the  formation  of  open  underground  spaces.  We  know 
full  well  that  any  such,  if  it  were  of  considerable  size, 
would  quickly  be  crushed  in  by  the  weight  of  the  over- 
lying rocks.  We  have,  indeed,  to  suppose  that  these  steam- 
impelled  lavas,  which  are  driven  toward  the  vent  whence 
they  are  to  go  forth  in  the  state  of  dust  or  fluid,  come 
underground  from  distances  away,  probably  from  beneath 
the  floors  of  the  sea  to  the  westward. 

Although  the  shores  of  the  Bay  of  Naples  have  re- 
mained in  general  with  unchanged  elevation  for  about  two 
thousand  years,  they  have  here  and  there  been  subjected 
to  slight  oscillations  which  are  most  likely  connected  with 
the  movement  of  volcanic  matter  toward  the  vents  where 
it  is  to  find  escape.  The  most  interesting  evidence  of  this 
nature  is  afforded  by  the  studies  which  have  been  made 
on  the  ruins  of  the  Temple  of  Serapis  at  Puzzuoli.  This 
edifice  was  constructed  in  pre-Christian  times  for  the  wor- 
ship of  the  Egyptian  god  Serapis,  whose  intervention  was 
sought  by  sick  people.  The  fact  that  this  divinity  of  the 
Nile  found  a  residence  in  this  region  shows  how  intimate 
was  the  relation  between  Eome  and  Egypt  in  this  ancient 
day.  The  Serapeium  was  built  on  the  edge  of  the  sea,  just 
above  its  level.  When  in  modern  days  it  began  to  be 
studied,  its  floor  was  about  on  its  original  level,  but  the 
few  standing  columns  of  the  edifice  afford  indubitable  evi- 
dence that  this  part  of  the  shore  has  been  lowered  to  the 
amount  of  twenty  feet  or  more  and  then  re-elevated.  The 
subsidence  is  proved  by  the  fact  that  the  upper  part  of  the 
columns  which  were  not  protected  by  the  debris  accumu- 
lated about  them  have  been  bored  by  certain  shellfish, 
known  as  Lithodomi,  which  have  the  habit  of  excavating 
shelters  in  soft  stone,  such  as  these  marble  columns  afford. 
At  present  the  floor  on  which  the  ruin  stands  appears  to  be 
gradually  sinking,  though  the  rate  of  movement  is  very  slow. 


288  OUTLINES  OF  THE  EARTPI'S  HISTORY. 

Another  evidence  that  the  ejections  may  travel  for  a 
great  distance  underground  on  their  way  to  the  vent  is 
afforded  by  the  fact  that  Vesuvius  and  ^tna,  though  near 
three  hundred  miles  apart,  appear  to  exchange  activities — • 
that  is,  their  periods  of  outbreak  are  not  simultaneous. 
Although  these  elements  of  the  chronology  of  the  two 
cones  may  be  accidental,  taken  with  similar  facts  derived 
from  other  fields,  they  appear  to  indicate  that  vents,  though 
far  separated  from  each  other,  may,  so  to  speak,  be  fed 
from  a  common  subterranean  source.  It  is  a  singular  fact 
in  this  connection  that  the  volcano  of  Stromboli,  though 
situated  between  these  two  cones,  is  in  a  state  of  almost 
incessant  activity.  This  probably  indicates  that  the  last- 
named  vent  derives  its  vapours  from  another  level  in  the 
earth  than  the  greater  cones.  In  this  regard  volcanoes 
probably  behave  like  springs,  of  which,  indeed,  they  may 
be  regarded  as  a  group.  The  reader  is  doubtless  aware  that 
hot  and  cold  springs  often  escape  very  near  together,  the 
difference  in  the  temperature  being  due  to  the  depth 
from  which  their  waters  come  forth. 

As  the  accidents  of  volcanic  explosion  are  of  a  nature 
to  be  very  damaging  to  man,  as  well  as  to  the  lower  orders 
of  Nature,  it  is  fit  that  we  should  note  in  general  the  effect 
of  the  Neapolitan  eruptions  on  the  history  of  civilization 
in  that  region.  As  stated  above,  the  first  Greek  settlements 
in  this  vicinity — those  on  the  island  of  Ischia — were  much 
disturbed  by  volcanic  outbreaks,  yet  the  island  became 
the  seat  of  a  permanent  and  prosperous  colony.  The  great 
eruption  of  79  probably  cost  many  hundred  lives,  and  led 
to  the  abandonment  of  two  considerable  cities,  which,  how- 
ever, could  at  small  cost  have  been  recovered  to  use.  Since 
that  day  various  eruptions  have  temporarily  desolated  por- 
tions of  the  territory,  but  only  in  very  small  fields  have 
the  ravages  been  irremediable.  Wliere  the  ground  was 
covered  with  dust,  it  has  in  most  places  been  again  tillable, 
and  so  rapid  is  the  decay  of  the  lavas  that  in  a  century 
after  their  flow  has  ceased  vines  can  in  most  cases  be 


THE  WORK  OF  UNDERGROUND  WATER.    289 

planted  on  their  surfaces.  The  city  of  Naples,  which  lies 
amid  the  vents,  though  not  immediately  in  contact  with 
any  of  them,  has  steadfastly  grown  and  prospered  from  the 
pre-Christian  times.  It  is  doubtful  if  any  lives  have  ever 
been  lost  in  the  city  in  consequence  of  an  eruption,  and  no 
great  inconvenience  has  been  experienced  from  them.  Now 
and  then,  after  a  great  ash  shower,  the  volcanic  dust  has 
to  be  removed,  but  the  labour  is  less  serious  than  that  im- 
posed on  many  northern  cities  by  a  snowstorm.  Through 
all  these  convulsions  the  tillage  of  the  district  has  been 
maintained.  It  has  ever  been  the  seat  of  as  rich  and 
profitable  a  husbandry  as  is  afforded  by  any  part  of  Italy. 
In  fact,  the  ash  showers,  as  they  import  fine  divided  rock 
very  rich  in  substances  necessary  for  the  growth  of  plants, 
have  in  a  measure  served  to  maintain  the  fertility  of  the 
soil,  and  by  this  action  have  in  some  degree  compensated 
for  the  injury  which  they  occasionally  inflict.  Comparing 
the  ravages  of  the  eruptions  with  those  inflicted  by  war, 
unnecessary  disease,  or  even  bad  politics,  and  we  see  that 
these  natural  accidents  have  been  most  merciful  to  man. 
Many  a  tyrant  has  caused  more  suffering  and  death  than 
has  been  inflicted  by  these  rude  operations  of  Nature. 

From  the  point  of  view  of  the  naturalist,  ^tna  is 
vastly  more  interesting  than  Vesuvius.  The  bulk  of  the 
cone  is  more  than  twenty  times  as  great  as  that  of  the 
Neapolitan  volcano,  and  the  magnitude  of  its  explosions, 
as  well  as  the  range  of  phenomena  which  they  exhibit, 
incomparably  greater.  It  happens,  however,  that  while 
human  history  of  the  recorded  kind  has  been  intimately 
bound  up  with  the  tiny  Vesuvian  cone,  partly  because  the 
relatively  slight  nature  of  its  disturbances  permitted  men 
to  dwell  beside  it,  the  larger  ^tna  has  expelled  culture 
from  the  field  near  its  vent,  and  has  done  the  greater  part 
of  its  work  in  the  vast  solitude  which  it  has  created.* 

*  In  part  the  excellent  record  of  Vesuvius  is  due  to  the  fact  that 
since  the  early  Christian  centuries  the  priests  of  St.  Januarius,  the  patron 


290  OUTLINES  OF  THE  EARTH'S  HISTORY. 

^tna  has  been  in  frequent  eruption  for  a  very  much 
longer  time  than  Vesuvius.  In  the  odes  of  Pindar,  in  the 
sixth  century  before  Christ,  we  find  records  of  eruptions. 
It  is  said  also  that  the  philosopher  Empedocles  sought 
fame  and  death  by  casting  himself  into  the  fiery  crater. 
There  has  thus  in  the  case  of  this  mountain  been  no  such 
long  period  of  repose  as  occurred  in  Vesuvius.  Though 
our  records  of  the  outbreaks  are  exceedingly  imperfect, 
they  serve  to  show  that  the  vent  has  maintained  its  activ- 
ity much  more  continuously  than  is  ordinarily  the  case 
with  volcanoes,  ^tna  is  characteristically  a  lava-yielding 
cone;  though  the  amount  of  dust  put  forth  is  large,  the 
ratio  of  the  fluid  rock  which  flows  away  from  the  crater  is 
very  much  greater  than  at  Vesuvius.  Nearly  half  the  cone, 
indeed,  may  be  composed  of  this  material.  Our  space  does 
not  permit  anything  like  a  consecutive  story  of  the  ^tnean 
eruptions  since  the  dawn  of  history,  or  even  a  full  account 
of  its  majestic  cone;  we  can  only  note  certain  features  of 
a  particularly  instructive  nature  which  have  been  re- 
marked by  the  many  able  men  who  have  studied  this  struc- 
ture and  the  effects  of  its  outbreak. 

The  most  important  feature  exhibited  by  ^tna  is  the 
vast  size  of  its  cone.  At  its  apex  its  height,  though  vari- 
able from  the  frequent  destruction  and  rebuilding  of  the 
crater  walls,  may  be  reckoned  as  about  eleven  thousand 
feet.  The  base  on  which  the  volcanic  material  lies  is 
probably  less  than  a  thousand  feet  above  the  sea,  so  that 
the  maximum  thickness  of  the  heap  of  volcanic  ejections 
is  probably  about  two  miles.  The  average  depth  of  this 
coating  is  probably  about  five  thousand  feet,  and,  as  the 
cone  has  an  average  diameter  of  about  thirty  miles,  we  may 
conclude  that  the  cone  now  contains  about  a  thousand 

of  Naples,  have  been  accustomed  to  carry  his  relics  in  procession  when- 
ever an  eruption  began.  The  cessation  of  the  outbreak  has  been  written 
down  to  the  credit  of  the  saint,  and  thus  we  are  provided  with  a  long 
story  of  the  successive  outbreaks. 


THE  WORK  OF  UNDERGROUND  WATER.         291 

cubic  miles  of  volcanic  materials.  Great  as  is  this  mass, 
it  is  only  a  small  part  of  the  ejected  material  which  has 
gone  forth  from  the  vent.  All  the  mater  which  in  its 
vaporous  state  went  forth  with  the  eruption,  the  other 
gases  and  vapours  thus  discharged,  have  disappeared.  So, 
too,  a  large  part  of  the  ash  and  much  of  the  lava  has 
been  swept  away  by  the  streams  which  drain  the  region, 
and  which  in  times  of  eruption  are  greatly  swollen  by  the 
accompanying  torrential  rains.  The  writer  has  estimated 
that  if  all  the  emanations  from  the  volcano — solid,  fluid, 
and  gaseous — could  be  heaped  on  the  cone,  they  would 
form  a  mass  of  between  two  and  three  thousand  cubic 
miles  in  contents.  Yet  notwithstanding  this  enormous 
outputting  of  earthy  matter,  the  earth  on  which  the 
^tnean  cone  has  been  constructed  has  not  only  failed  to 
sink  down,  but  has  been  in  process  of  continuous,  slow 
uprising,  which  has  lifted  the  surface  more  than  a  thou- 
sand feet  above  the  level  which  it  had  at  the  time  when 
volcanic  action  began  in  this  field.  Here,  even  more  clearly 
than  in  the  case  of  Vesuvius,  we  see  that  the  materials 
driven  forth  from  the  crater  are  derived  not  from  just 
beneath  its  foundation,  but  from  a  distance,  from  realms 
which  in  the  case  of  this  insular  volcano  are  beneath  the 
sea  floors.  It  is  certain  that  here  the  migration  of  rock 
matter,  impelled  by  the  expansion  of  its  contained  water 
toward  the  vent,  has  so  far  exceeded  that  which  has  been 
discharged  through  the  crater  that  an  uprising  of  the  sur- 
face such  as  we  have  observed  has  been  brought  about. 

There  are  certain  peculiarities  of  Mount  ^Etna  which 
are  due  in  part  to  its  great  size  and  in  part  to  the  climatal 
conditions  of  the  region  in  which  it  lies.  The  upper  part 
of  the  mountain  in  winter  is  deeply  snow-clad;  the  frozen 
water  often,  indeed,  forms  great  drifts  in  the  gorges  near 
the  summit.  Here  it  has  occasionally  happened  that  a 
layer  of  ashes  has  deeply  buried  the  mass,  so  that  it  has 
been  preserved  for  years,  becoming  gradually  more  in- 
closed by  the  subsequent  eruptions.     At  one  point  where 


292  OUTLINES  OP  THE  EAllTH'S  HISTORY. 

this  compact  snow — which  has,  indeed,  taken  on  the  form 
of  ice — has  been  revealed  to  view,  it  has  been  quarried  and 
conveyed  to  the  towns  upon  the  seacoast.  It  is  likely  that 
there  are  many  such  masses  of  ice  inclosed  between  the 
ash  layers  in  the  upper  part  of  the  mountain,  where,  owing 
to  the  height,  the  climate  is  very  cold.  This  curious  fact 
shows  how  perfect  a  non-conductor  the  ash  beds  of  a  vol- 
cano are  to  protect  the  frozen  water  from  the  heat  of  the 
rocks  about  the  crater. 

The  furious  rains  which  beset  the  mountain  in  times 
of  great  eruptions  excavate  deep  channels  on  its  sides.  The 
lava  outbreaks  which  attend  almost  every  eruption,  and 
which  descend  from  the  base  of  the  cinder  cone  at  the 
height  of  from  five  to  eight  thousand  feet  above  the  sea, 
naturally  find  their  Avay  into  these  channels,  where  they 
course  in  the  manner  of  rivers  until  the  lower  and  less 
valleyed  section  of  the  cone  is  reached. 

Such  a  lava  flow  naturally  begins  to  freeze  on  the  sur- 
face, the  lava  at  first  becoming  viscid,  much  in  the  manner 
of  cream  on  the  surface  of  milk.  Urged  along  by  the 
more  fluid  lava  underneath,  this  viscid  coating  takes  a 
ropy  or  corrugated  form.  As  the  freezing  goes  deeper, 
a  firm  stone  roof  may  be  formed  across  the  gorge, 
which,  when  the  current  of  lava  ceases  to  flow  from  the 
crater,  permits  the  lower  part  of  the  stream  to  drain 
away,  leaving  a  long  cavern  or  series  of  caves  ex- 
tending far  up  the  cone.  The  nature  of  this  action  is 
exactly  comparable  to  that  which  we  may  observe  when 
on  a  frosty  morning  after  rain  we  may  find  the  empty 
channels  which  were  occupied  by  rills  of  water  roofed  over 
with  ice;  the  ice  roofs  are  temporary,  while  those  of  lava 
may  endure  for  ages.  Some  of  these  lava-stream  caves 
have  been  disclosed,  in  the  manner  of  ordinary  caverns,  by 
the  falling  of  their  roofs;  but  the  greater  part  are  natu- 
rally hidden  beneath  the  ever-increasing  materials  of  the 
cone. 

The  lava-stream  caves  of  ^tna  are  not  only  interesting 


THE  WORK  OF  UNDERGROUND  WATER.    293 

because  of  their  peculiarities  of  form,  which  we  shall  not 
undertake  to  describe,  but  also  for  the  reason  that  they 
help  us  to  account  for  a  very  peculiar  feature  in  the  his- 
tory of  the  great  cone.  On  the  slopes  of  the  volcano, 
below  the  upper  cindery  portion,  there  are  several  hun- 
dred lesser  cones,  varying  from  a  few  score  to  seven  hun- 
dred feet  in  height.  Each  of  these  has  its  appropriate 
crater,  and  has  evidently  been  the  seat  of  one  or  more 
eruptions.  As  the  greater  part  of  these  cones  are  ancient, 
many  of  them  being  almost  effaced  by  the  rain  or  buried 
beneath  the  ejections  which  have  surrounded  their  bases 
since  the  time  they  were  formed,  we  are  led  to  believe 
that  many  thousands  of  them  have  been  formed  during 
the  history  of  the  volcano.  The  history  of  these  sub- 
sidiary cones  appears'  to  be  connected  with  the  lava  caves 
noted  above.  These  caverns,  owing  to  the  irregularities 
of  their  form,  contain  water.  They  are,  in  fact,  natural 
cisterns,  where  the  abundant  rainfall  of  the  mountain  finds 
here  and  there  storage.  When,  during  the  throes  of  an  erup- 
tion, dikes  such  as  we  know  often  to  penetrate  the  moun- 
tain, are  riven  outward  from  the  crater  through  the  mass 
of  the  cone,  and  filled  with  lava,  the  heated  rock  must 
often  come  in  contact  with  these  masses  of  buried  water. 
The  result  of  this  would  inevitably  be  the  local  genera- 
tion of  steam  at  a  high  temperature,  which  would  force 
its  way  out  in  a  brief  but  vigorous  eruption,  such  as  has 
been  observed  to  take  place  when  these  peripheral  vol- 
canoes are  formed.  Sometimes  it  has  happened  that  after 
the  explosion  the  lava  has  found  its  way  in  a  stream  from 
the  fissure  thus  opened.  That  this  explanation  is  suffi- 
cient is  in  a  measure  shown  by  observations  on  certain 
effects  of  lava  flows  from  Vesuvius.  The  writer  was  in- 
formed by  a  very  judicious  observer,  a  resident  of  Naples, 
who  had  interested  himself  in  the  phenomena  of  that  vol- 
cano, that  the  lava  streams  when  they  penetrated  a  cis- 
tern, such  as  they  often  encounter  in  passing  over  villages 
or  fa,rmsteads,  vaporized  the  water,  and  gave  rise,  through 


294:  OUTLINES  OF  THE  EARTH'S  HISTORY. 

the  action  of  the  steam,  to  small  temporary  cones,  which, 
though  generally  washed  away  hy  the  further  flow  of  the 
liquid  rock,  are  essentially  like  those  which  we  find  on 
^Etna.  Such  subsidiary,  or,  as  they  are  sometimes  called, 
parasitic  cones,  are  known  about  other  volcanoes,  but  no- 
where are  they  so  characteristic  as  on  the  flanks  of  that 
wonderful  volcano. 

A  very  conspicuous  feature  in  the  vEtnean  cone  con- 
sists of  a  great  valley  known  as  the  Val  del  Bove,  or  Bull 
Hollow,  which  extends  from  the  base  of  the  modern  and 
ever-changeable  cinder  cone  down  the  flanks  of  the  older 
structure  to  near  its  base.  This  valley  has  steep  sides,  in 
places  a  thousand  or  more  feet  high,  and  has  evidently 
been  formed  by  the  down-settling  of  portions  of  the  cone 
which  were  left  without  support  by  the  withdrawal  from 
beneath  them  of  materials  cast  forth  in  a  time  of  ex- 
plosion. In  an  eruption  this  remarkable  valley  was  the 
seat  of  a  vast  water  flood,  the  fluid  being  cast  forth  from 
the  crater  at  the  beginning  of  the  explosion.  In  the 
mouths  of  this  and  other  volcanoes,  after  a  long  period  of 
repose,  great  quantities  of  water,  gathering  from  rains  or 
condensed  from  the  steam  which  slowly  escapes  from  these 
openings,  often  pours  like  a  flood  down  the  sides  of  the 
mountains.  In  the  great  eruption  of  Galongoon,  in  Java, 
such  a  mass  of  water,  cast  forth  by  a  terrific  explosion, 
mingled  with  ashes,  so  that  the  mass  formed  a  thick  mud, 
was  shot  forth  with  such  energy  that  it  ravaged  an  area 
nearly  eighty  miles  in  diameter,  destroying  the  forests 
and  their  wild  inhabitants,  as  well  as  the  people  Avho  dwelt 
within  the  range  of  the  amazing  disaster.  So  powerfully 
was  this  water  driven  from  the  crater  that  the  districts 
immediately  at  the  base  of  the  cone  were  in  a  manner 
overshot  by  the  vast  stream,  and  escaped  with  relatively 
little  injury. 

When  it  comes  forth  from  the  base  of  the  cinder  cone, 
or  from  one  of  the  small  peripheral  craters,  the  lava 
stream  usually  appears  to  be  white  hot,  and  to  flow  with 


THE  WORK  OF   UNDERGROUND  WATER.         295 

almost  the  ease  of  water.  It  does  not  really  have  that 
measure  of  fluidity;  its  condition  is  rather  that  of  thin 
paste;  but  the  great  weight  of  the  material — near  two 
and  a  half  times  that  of  water — causes  the  movement  down 
the  slope  to  be  speedy.  The  central  portion  of  the  lava 
stream  long  retains  its  high  temperature;  but  the  surface, 
cooling,  is  first  converted  into  a  tough  sheet,  which,  though 
it  may  bend,  can  hardly  be  said  to  flow.  Further  harden- 
ing converts  these  outlying  portions  of  the  current  into 
hard,  glassy  stone,  which  is  broken  into  fragments  in  a 
way  resembling  the  ice  on  the  surface  of  a  river.  It  thus 
comes  about  that  the  advancing  front  of  the  lava  stream 
becomes  covered,  and  its  motion  hindered  by  the  frozen 
rock,  until  the  rate  of  ongoing  may  not  exceed  a  few  feet 
an  hour,  and  the  appearance  is  that  of  a  heap  of  stone 
slowly  rolling  down  a  slope.  Now  and  then  a  crevice  is 
formed,  through  which  a  thin  stream  of  liquid  lava  pours 
forth,  but  the  material,  having  already  parted  with  much 
of  its  heat,  rapidly  cools,  and  in  turn  becomes  covered  with 
the  coating  of  frozen  fragments.  In  this  state  of  the 
stream  the  lava  flow  stands  on  all  sides  high  above  the 
slope  which  it  is  traversing;  it  is,  in  fact,  walled  in  by  its 
own  solidified  parts,  though  it  is  urged  forward  by  the 
contribution  which  continues  to  flow  in  the  under  arches. 
In  this  state  of  the  movement  trifling  accidents,  or  even 
human  interference,  may  direct  the  current  this  way  or 
that. 

Some  of  the  most  interesting  chapters  in  the  history 
of  ^tna  relate  to  the  efforts  of  the  people  to  turn  these 
slow-moving  streams  so  that  their  torrents  might  flow  into 
wilderness  places  rather  than  over  the  fields  and  towns. 
In  the  great  flow  of  1669,  which  menaced  the  city  of 
Catania,  a  large  place  on  the  seashore  to  the  southeast 
of  the  cone,  a  public-spirited  citizen,  Seiior  Papallardo, 
protecting  himself  and  his  servants  w^ith  clothing  made 
of  hides,  and  with  large  shields,  set  forth  armed  with  great 
hooks  with  the  purpose  of  diverting  the  course  of  the  lava 
20 


296  OUTLINES  OF  THE  EAllTH'S  HISTORY. 

mass.  He  succeeded  in  pulling  away  the  stones  on  the 
flank  of  the  stream,  so  that  a  flow  of  the  molten  rock  was 
turned  in  another  direction.  The  expedient  would  prob- 
ably have  been  successful  if  he  had  been  allowed  to  con- 
tinue his  labours;  but  the  inhabitants  of  a  neighbouring 
village,  which  was  threatened  by  the  off-shooting  current 
which  Papallardo  had  created,  took  up  arms  and  drove 
him  and  his  retainers  away.  The  flow  continued  until  it 
reached  Catania.  The  people  made  haste  to  build  the  city 
walls  on  the  side  of  danger  higher  than  it  was  before,  but 
the  tide  mounted  over  its  summit. 

Although  the  lavas  which  come  forth  from  the  volcano 
evidently  have  a  high  temperature,  their  capacity  for  melt- 
ing other  rocks  is  relatively  small.  They  scour  these  rocks, 
because  of  their  weight,  even  more  energetically  than  do 
powerful  torrents  of  water,  but  they  are  relatively  in- 
effective in  melting  stone.  On  ^tna  and  elsewhere  we 
may  often  observe  lavas  which  have  flowed  through  for- 
ests. When  the  tide  of  molten  rock  has  passed  by,  the 
trees  may  be  found  charred  but  not  entirely  burned  away; 
even  stems  a  few  inches  in  diameter  retain  strength  enough 
to  uphold  considerable  fringes  and  clots  of  the  lava  which 
has  clung  to  them.  These  facts  bear  out  the  conclusion 
that  the  fluidity  of  the  heated  stone  depends  in  consider- 
able measure  on  the  water  which  is  contained,  either  in 
its  fluid  or  vaporous  state,  between  the  particles  of  the 
material. 

If  we  consider  the  Italian  volcanoes  as  a  whole,  we 
find  that  they  lie  in  a  long,  discontinuous  line  extending 
from  the  northern  part  of  the  valley  of  the  Po,  within 
sight  of  the  Alps,  to  JEtna,  and  in  subterranean  cones  per- 
haps to  the  northern  coast  of  Africa.  At  the  northern 
end  of  the  line  we  have  a  beautiful  group  of  extinct  vol- 
canoes, known  as  the  Eugean  Mountains.  Thence  south- 
ward to  southern  Tuscany  craters  are  wanting,  but  there 
is  evidence  of  fissures  in  the  earth  which  give  forth  ther- 
mal waters.    From  southern  Tuscany  southward  through 


THE  WORK  OF  UNDERGROUND  WATER.         297 

Eome  to  Naples  there  are  many  extinct  craters,  none  of 
which  have  been  active  in  the  historic  period.  From 
Naples  southward  the  cones  of  this  system,  about  a  dozen 
in  number,  are  on  islands  or  close  to  the  margin  of  the 
sea.  It  is  a  noteworthy  fact  that  the  greater  part  of  these 
shore  or  insular  vents  have  been  active  since  the  dawn  of 
history;  several  of  them  frequently  and  furiously  so,  while 
none  of  those  occupying  an  inland  position  have  been  the 
seat  of  explosions.  This  is  a  striking  instance  going  to 
show  the  relation  of  these  processes  to  conditions  which 
are  brought  about  on  the  sea  bottom. 

^tna  is,  as  we  have  noticed,  a  much  more  powerful 
volcano  than  Vesuvius.  Its  outbreaks  are  more  vigorous, 
its  emanations  vastly  greater  in  volume,  and  the  mass  of  its 
constructions  many  times  as  great  as  those  accumulated 
in  any  other  European  cone.  There  are,  however,  a  num- 
ber of  volcanoes  in  the  world  which  in  certain  features 
surpass  vEtna  as  much  as  that  crater  does  Vesuvius.  Of 
these  we  shall  consider  but  two — Skaptar  Jokul,  of  Ice- 
land, remarkable  for  the  volume  of  its  lava  flow,  and 
Krakatoa,  an  island  volcano  between  Java  and  Sumatra, 
which  was  the  seat  of  the  greatest  explosion  of  which  we 
have  any  record. 

The  whole  of  Iceland  may  be  regarded  as  a  volcanic 
mass  composed  mainly  of  lavas  and  ashes  which  have 
been  thrown  up  by  a  group  of  volcanoes  lying  near  the 
northern  end  of  the  long  igneous  axis  which  extends 
through  the  centre  of  the  Atlantic.  The  island  has  been 
the  seat  of  numerous  eruptions;  in  fact,  since  its  settle- 
ment by  the  Northmen  in  1070  its  sturdy  inhabitants 
have  been  almost  as  much  distressed  by  the  calamities 
which  have  come  from  the  internal  heat  as  they  have  been 
by  the  enduring  external  cold.  They  have,  indeed,  been 
between  frost  and  fire.  The  greatest  recorded  eruption  of 
Iceland  occurred  in  1783,  when  the  volcano  of  Skaptar, 
near  the  southern  border  of  the  island,  poured  forth,  first, 
a  vast  discharge  of  dust  and  ashes,  and  afterward  in  the 


298  OUTLINES  OF  THE  EARTH'S  HISTORY. 

languid  state  of  eruption  inundated  a  series  of  valleys  with 
the  greatest  lava  flow  of  which  we  have  any  written  record. 
The  dust  poured  forth  into  the  upper  air,  being  finely  di- 
vided and  in  enormous  quantity,  floated  in  the  air  for 
months,  giving  a  dusky  hue  to  the  skies  of  Europe,  which 
led  the  common  people  and  many  of  the  learned  to  fear 
that  the  wrath  of  God  was  upon  them,  and  that  the  day 
of  judgment  was  at  hand.  Even  the  poet  Cowper,  a  man 
of  high  culture  and  education,  shared  in  this  unreasonable 
view. 

The  lava  flow  in  this  eruption  filled  one  of  the  consid- 
erable valleys  of  the  island,  drying  up  the  river,  and  inun- 
dating the  plains  on  either  side.  Estimates  which  have 
been  made  as  to  the  volume  of  this  flow  appear  to  indicate 
that  it  may  have  amounted  to  more  than  the  bulk  of  the 
Mont  Blanc. 

This  great  eruption,  by  the  direct  effect  of  the  calam- 
ity, and  by  the  famine  due  to  the  ravaging  of  the  fields  and 
the  frightening  of  the  fish  from  the  shores  which  it  in- 
duced, destroyed  nearly  one  fifth  of  the  Icelandic  people. 
It  is,  in  fact,  to  be  remembered  as  one  of  the  three  or  four 
most  calamitous  eruptions  of  which  we  have  any  account, 
and,  from  the  point  of  view  of  lava  flow,  the  greatest  in 
history. 

Just  a  hundred  years  after  the  great  Skaptar  eruption, 
which  darkened  the  skies  of  Europe,  the  island  of  Kraka- 
toa,  an  isle  formed  by  a  small  volcano  in  the  straits  of  Java, 
was  the  seat  of  a  vapour  explosion  which  from  its  intensity 
is  not  only  unparalleled,  but  almost  unapproached  in  all 
accounts  of  such  disturbances.  Krakatoa  had  long  been 
recognised  as  a  volcanic  isle;  it  is  doubtful,  however,  if 
it  had  ever  been  seen  in  eruption  during  the  three  cen- 
turies or  more  since  European  ships  began  to  sail  by  it 
until  the  month  of  May  of  the  year  above  mentioned. 
Then  an  outbreak  of  what  may  be  called  ordinary  vio- 
lence took  place,  which  after  a  few  days  so  far  ceased  that 
observers  landed  and  took  account  of  the  changes  which 


THE  WORK  OF  UNDERGROUND  WATER.    299 

the  convulsion  had  brought  about.  For  about  three  months 
there  were  no  further  signs  of  activity,  but  on  the  29th  of 
August  a  succession  of  vast  explosions  took  place,  which 
blew  away  a  great  part  of  the  island,  forming  in  its  place  a 
submarine  crater  two  or  three  miles  in  diameter,  creating 
world-wide  disturbances  of  sea  and  air.  The  sounds  of  the 
outbreak  were  heard  at  a  distance  of  sixteen  hundred  miles 
away.  The  waves  of  the  air  attendant  on  the  explosion 
ran  round  the  earth  at  least  once,  as  was  distinctly  indi- 
cated by  the  self-recording  barometers;  it  is  possible,  in- 
deed, that,  crossing  each  other  in  their  east  and  west 
courses,  these  atmospheric  tides  twice  girdled  the  sphere. 
In  effect,  the  air  over  the  crater  was  heaved  up  to  the 
height  of  some  tens  of  thousands  of  feet,  and  thence  rolled 
off  in  great  circular  waves,  such  as  may  be  observed  in  a 
pan  of  milk  when  a  sharp  blow  pushes  the  bottom  upward. 

The  violent  stroke  delivered  to  the  waters  of  the  sea 
created  a  vast  wave,  which  in  the  region  where  it  origi- 
nated rolled  upon  the  shores  with  a  surf  wall  fifty  or  more 
feet  high.  In  a  few  minutes  about  thirty  thousand  people 
were  overwhelmed.  The  wave  rolled  on  beyond  its  de- 
structive limits  much  in  the  manner  of  the  tide;  its  influ- 
ence was  felt  in  a  sharp  rise  and  fall  of  the  waters  as  far 
as  the  Pacific  coast  of  North  America,  and  was  indicated 
by  the  tide  gauges  in  the  Atlantic  as  far  north  as  the  coast 
of  Europe. 

Owing  to  the  violence  of  the  eruption,  Krakatoa  poured 
forth  no  lava,  but  the  dust  and  ashes  which  ascended  into 
the  air — or,  in  other  words,  the  finely  divided  lava  which 
escaped  into  the  atmosphere — probably  amounted  in  bulk 
to  more  than  twenty  cubic  miles.  The  coarser  part  of  this 
material,  including  much  pumice,  fell  upon  the  seas  in  the 
vicinity,  where,  owing  to  its  lightness,  it  was  free  to  drift 
in  the  marine  currents  far  and  wide  throughout  the  oceanic 
realm.  The  finer  particles,  thrown  high  into  the  air,  per- 
haps to  the  height  of  nearly  a  hundred  thousand  feet — cer- 
tainly to  the  elevation  of  more  than  half  this  amount — 


300  OUTLINES  OF  THE  EARTH'S  HISTORY. 

drifted  far  and  wide  in  the  atmosphere,  so  that  for  years 
the  air  of  all  regions  was  clouded  by  it,  the  sunrise  and 
sunset  having  a  peculiar  red  glow,  which  the  dust  particles 
produce  by  the  light  which  they  reflect.  In  this  period,  at 
all  times  when  the  day  was  clear,  the  sun  appeared  to  be 
surrounded  by  a  dusky  halo.  In  time  the  greater  part  of 
this  dust  was  drawn  down  by  gravity,  some  portion  of  it 
probably  falling  on  every  square  foot  of  the  earth.  Since 
the  disappearance  of  the  characteristic  phenomena  which 
it  produced  in  the  atmosphere,  European  observers  have 
noted  the  existence  of  faint  clouds  lying  in  the  upper  part 
of  the  air  at  the  height  of  a  hundred  miles  or  more  above 
the  surface.  These  clouds,  which  were  at  first  distinctly 
visible  in  the  earliest  stage  of  dawn  and  in  the  latest  period 
of  the  sunset  glow,  seemed  to  be  in  rapid  motion  to  the 
eastward,  and  to  be  mounting  higher  above  the  earth.  It 
has  been  not  unreasonably  supposed  that  these  sliining 
clouds  represent  portions  of  the  finest  dust  from  Krakatoa, 
which  has  been  thrown  so  far  above  the  earth's  attraction 
that  it  is  separating  itself  from  the  sphere.  If  this  view  be 
correct,  it  seems  likely  that  we  may  look  to  great  volcanic 
explosions  as  a  source  whence  the  dustlike  particles  which 
people  the  celestial  spaces  may  have  come.  They  may,  in 
a  word,  be  due  to  volcanic  explosions  occurring  on  this  and 
other  celestial  spheres. 

The  question  suggested  above  as  to  the  possibility  of 
volcanic  ejections  throwing  matter  from  the  earth  beyond 
the  control  of  its  gravitative  energy  is  one  of  great  scien- 
tific interest.  Computations  (not  altogether  trustworthy) 
show  that  a  body  leaving  the  earth's  surface  under  the 
conditions  of  a  cannon  ball  fired  vertically  upward  would 
have  to  possess  a  velocity  at  the  start  of  at  least  seven 
miles  a  second  in  order  to  go  free  into  space.  It  would 
at  first  sight  seem  that  we  should  be  able  to  reckon  whether 
volcanoes  can  propel  earth  matter  upward  with  this  speed. 
In  fact,  however,  sufficient  data  are  not  obtainable;  we 
only  know  in  a  general  way  that  the  column  of  vapour 


THE  WORK  OF  UNDERGROUND  WATER.    301 

rises  to  the  height  of  thirty  or  forty  thousand  feet,  and 
this  in  eruptions  of  no  great  magnitude.  In  an  accident 
such  as  that  at  Krakatoa,  even  if  an  observer  were  near 
enough  to  see  clearly  what  was  going  on,  the  chance  of 
his  surviving  the  disturbance  would  be  small.  Moreover, 
the  ascending  vapours,  owing  to  their  expansion  of  the 
steam  in  the  column,  begin  to  fly  out  sideways  on  its  pe- 
riphery, so  that  the  upper  part  of  the  central  section  in 
the  discharge  is  not  visible  from  the  earth. 

It  is  in  the  central  section  of  the  uprushing  mass,  if 
anywhere,  that  the  dust  might  attain  the  height  necessary 
to  put  it  beyond  the  earth's  attraction,  bringing  it  fairly 
into  the  realm  of  the  solar  system,  or  to  the  position  where 
its  own  motion  and  the  attraction  of  the  other  spheres 
would  give  it  an  independent  orbital  movement  about 
the  sun,  or  perhaps  about  the  earth.  We  can  only  say  that 
observations  on  the  height  of  volcanic  ejections  are  ex- 
tremely desirable;  they  can  probably  only  be  made  from 
a  balloon.  An  ascension  thus  made  beyond  the  cloud 
disk  which  the  eruption  produces  might  bring  the  ob- 
server where  he  could  discern  enough  to  determine  the 
matter.  Although  the  movements  of  the  rocky  particles 
could  not  be  observed,  the  colour  which  they  would  give 
to  the  heavens  might  tell  the  story  which  we  wish  to  know. 
There  is  evidence  that  large  masses  of  stone  hurled  up 
by  volcanic  eruption  have  fallen  seven  miles  from  the 
base  of  the  cone.  Assuming  that  the  masses  went  straight 
upward  at  the  beginning  of  their  ascent,  and  that  they  were 
afterward  borne  outwardly  by  the  expansion  of  the  column, 
computations  which  have  a  general  but  no  absolute  value 
appear  to  indicate  that  the  masses  attained  a  height  of 
from  thirty  to  fifty  miles,  and  had  an  initial  velocity  which, 
if  doubled,  might  have  carried  them  into  space. 

Last  of  all,  we  shall  note  the  conditions  which  attend 
the  eruptions  of  submarine  volcanoes.  Such  explosions 
have  been  observed  in  but  a  few  instances,  and  only  in 
those  cases  where  there  is  reason  to  believe  that  the  crater 


302  OUTLINES  OF  THE  EARTH'S  HISTORY. 

at  the  time  of  its  explosion  had  attained  to  within  a  few 
hundred  feet  of  the  sea  level.  In  these  cases  the  ejections, 
never  as  yet  observed  in  the  state  of  lava,  but  in  the  con- 
dition of  dust  and  pumice,  have  occasionally  formed  a  low 
island,  which  has  shortly  been  washed  away  by  the  waves. 
Knowing  as  we  do  that  volcanoes  abound  on  the  sea  floor, 
the  question  why  w^e  do  not  oftener  see  their  explosions 
disturbing  the  surface  of  the  waters  is  very  interesting, 
but  not  as  yet  clearly  explicable.  It  is  possible,  however, 
that  a  volcanic  discharge  taking  place  at  the  depth  of  sev- 
eral thousand  feet  below  the  surface  of  the  water  would 
not  be  able  to  blow  the  fluid  aside  so  as  to  open  a  pipe 
to  the  surface,  but  would  expend  its  energy  in  a  hidden 
manner  near  the  ocean  floor.  The  vapours  would  have  to 
expand  gradually,  as  they  do  in  passing  up  through  the 
rock  pipe  of  a  volcano,  and  in  their  slow  upward  passage 
might  be  absorbed  by  the  water.  The  solid  materials 
thrown  forth  would  in  this  case  necessarily  fall  close  about 
the  vent,  and  create  a  very  steep  cone,  such,  indeed,  as  we 
find  indicated  by  the  soundings  off  certain  volcanic  islands 
which  appear  only  recently  to  have  overtopped  the  level 
of  the  waters. 

As  will  be  seen,  though  inadequately  from  the  dia- 
grams of  Vesuvius,  volcanic  cones  have  a  regularity  and 
symmetry  of  form  far  exceeding  that  afforded  by  the  out- 
lines of  any  other  of  the  earth's  features.  Where,  as  is 
generally  the  case,  the  shape  of  the  cone  is  determined  by 
the  distribution  of  the  falling  cinders  or  divided  lava  which 
constitutes  the  mass  of  most  cones,  the  slope  is  in  general 
that  known  as  a  catenary  curve — i.  e.,  the  line  formed  by 
a  chain  hanging  between  two  points  at  some  distance  from 
the  vertical.  It  is  interesting  to  note  that  this  graceful 
outline  is  a  reflection  or  consequence  of  the  curve  described 
by  the  uprushing  vapour.  The  expansion  in  the  ascending 
column  causes  it  to  enlarge  at  a  somewhat  steadfast  rate, 
while  the  speed  of  the  ascent  is  ever  diminishing.  Pre- 
cisely the  same  action  can  be  seen  in  the  like  rush  of  steam 


THE  WORK  OF  UNDERGROUND  WATER.    303 

and  other  gases  and  vapours  from  tlie  cannon's  mouth;  only 
in  the  case  of  the  gun,  even  of  the  greatest  size,  we  can 
not  trace  the  movement  for  more  than  a  few  hundred  feet. 
In  this  column  of  ejection  the  outward  movement  from 
the  centre  carries  the  bits  of  lava  outwardly  from  the 
centre  of  the  shaft,  so  that  when  they  lose  their  ascending 
velocity  they  are  drawn  downward  upon  the  flanks  of  the 
cone,  the  amount  falling  upon  each  part  of  that  surface 
being  in  a  general  way  proportional  to  the  thickness  of 
the  vaporous  mass  from  which  they  descend.  The  result 
is,  that  the  thickest  part  of  the  ash  heap  is  formed  on  the 
upper  part  of  the  crater,  from  which  point  the  deposit  fades 
away  in  depth  in  every  direction.  In  a  certain  measure 
the  concentration  toward  the  centre  of  the  cone  is  brought 
about  by  the  draught  of  air  which  moves  in  toward  the 
ascending  column. 

Although,  in  general,  ejections  of  volcanic  matter  take 
place  through  cones,  that  being  the  inevitable  form  pro- 
duced by  the  escaping  steam,  very  extensive  outpourings 
of  lava,  ejections  which  in  mass  probably  far  exceed  those 
thrown  forth  through  ordinary  craters,  are  occasionally 
poured  out  through  fissures  in  the  earth's  crust.  Thus  in 
Oregon,  Idaho,  and  Washington,  in  eastern  Europe,  in 
southern  India,  and  at  some  other  points,  vast  flows,  which 
apparently  took  place  from  fissures,  have  inundated  great 
realms  with  lava  ejections.  The  conditions  which  appear 
to  bring  about  these  fissure  eruptions  of  lava  are  not  yet 
well  understood.  A  provisional  and  very  probable  account 
of  the  action  can  be  had  in  the  hypothesis  which  will  now 
be  set  forth. 

Where  any  region  has  been  for  a  long  time  the  seat  of 
volcanic  action,  it  is  probable  that  a  large  amount  of  rock 
in  a  more  or  less  fluid  condition  exists  beneath  its  surface. 
Although  the  outrushing  steam  ejects  much  of  this  molten 
material,  there  are  reasons  to  suppose  that  a  yet  greater 
part  lies  dormant  in  the  underground  spaces.  Thus  in  the 
case  of  J^tna  we  have  seen  that,  though  some  thousands 


304:  OUTLINES  OF  THE  EARTH'S  HISTORY. 

of  miles  of  rock  matter  have  come  forth,  the  base  of  the 
cone  has  been  uplifted,  probably  by  the  moving  to  that 
region  of  more  or  less  fluid  rock.  If  now  a  region  thus 
underlaid  by  what  we  may  call  incipient  lavas  is  subjected 
to  the  peculiar  compressive  actions  which  lead  to  mountain- 
building,  we  should  naturally  expect  that  such  soft  mate- 
rial would  be  poured  forth,  possibly  in  vast  quantities 
through  fault  fissures,  which  are  so  readily  formed  in  all 
kinds  of  rock  when  subject  to  irregular  and  powerful 
strains,  such  as  are  necessarily  brought  about  when  rocks 
are  moved  in  mountain-making.  The  great  eruptions 
which  formed  the  volcanic  table-lands  on  the  west  coast 
of  North  America  appear  to  have  owed  the  extrusion  of 
their  materials  to  mountain-building  actions.  This  seems 
to  have  been  the  case  also  in  some  of  those  smaller  areas 
where  fissure  flows  occur  in  Europe.  It  is  likely  that  this 
action  will  explain  the  greater  part  of  these  massive  erup- 
tions. 

It  need  not  be  supposed  that  the  rock  beneath  these 
countries,  which  when  forced  out  became  lava,  was  neces- 
sarily in  the  state  of  perfect  fluidity  before  it  was  forced 
through  the  fissures.  Situated  at  great  depth  in  the  earth, 
it  was  under  a  pressure  so  great  that  its  particles  may 
have  been  so  brought  together  that  the  material  was  essen- 
tially solid,  though  free  to  move  under  the  great  strains 
which  affected  it,  and  acquiring  temperature  along  with 
the  fluidity  which  heat  induces  as  it  was  forced  along  by 
the  mountain-building  pressure.  As  an  illustration  of  how 
materials  may  become  highly  heated  when  forced  to  move 
particle  on  particle,  it  may  be  well  to  cite  the  case  in  which 
the  iron  stringpiece  on  top  of  a  wooden  dam  near  Holyoke, 
Mass.,  was  affected  when  the  barrier  went  away  in  a  flood. 
The  iron  stringer,  being  very  well  put  together,  was,  it  is 
said,  drawn  out  by  the  strain  until  it  became  sensibly  red- 
dened by  the  motion  of  its  particles,  and  finally  fell  hiss- 
ing into  the  waters  below.  A  like  heating  is  observable 
when  metal  is  drawn  out  in  making  wire.    Thus  a  mass  of 


THE  WORK  OF  UNDERGROUND  WATER.    305 

imperfectly  fluid  rock  might  in  a  forced  journey  of  a  few 
miles  acquire  a  decided  increase  of  temperature. 

Although  the  most  striking  volcanic  action — all  such 
phenomena,  indeed,  as  commonly  receives  the  name — is 
exhibited  finally  on  the  earth's  surface,  a  great  deal  of 
work  which  belongs  in  the  same  group  of  geological  actions 
is  altogether  confined  to  the  deep-lying  rock,  and  leads  to 
the  formation  of  dikes  which  penetrate  the  strata,  but  do 
not  rise  to  the  open  air.  We  have  already  noted  the  fact 
that  dikes  abound  in  the  deeper  parts  of  volcanic  cones, 
though  the  fissures  into  which  they  find  their  way  are  sel- 
dom riven  up  to  the  surface.  In  the  same  way  beneath 
the  ground  in  non-volcanic  countries  we  may  discover  at 
a  great  depth  in  the  older,  much-changed  rock  a  vast  num- 
ber of  these  crevices,  varying  from  a  few  inches  to  a  hun- 
dred feet  or  more  in  width,  which  have  been  filled  with 
lavas,  the  rock  once  molten  having  afterward  cooled.  In 
most  cases  these  dikes  are  disclosed  to  us  through  the 
down-wearing  of  the  earth  that  has  removed  the  beds  into 
which  the  dikes  did  not  penetrate,  thus  disclosing  the 
realm  in  which  the  disturbances  took  place. 

Where,  as  is  occasionally  the  case  in  deep  mines,  or 
on  some  bare  rocky  cliff  of  great  height,  we  can  trace  a 
dike  in  its  upward  course  through  a  long  distance,  we  find 
that  we  can  never  distinctly  discover  the  lower  point  of  its 
extension.  No  one  has  ever  seen  in  a  clear  way  the  point 
of  origin  of  such  an  injection.  We  can,  however,  often 
follow  it  upward  to  the  place  where  there  was  no  longer 
a  rift  into  which  it  could  enter.  In  its  upward  path  the 
molten  matter  appears  generally  to  have  followed  some 
previously  existing  fracture,  a  joint  plane  or  a  fault,  which 
generally  runs  through  the  rocks  on  those  planes.  We  can 
observe  evidence  that  the  material  was  in  the  state  of  igne- 
ous fluidity  by  the  fact  that  it  has  baked  the  country  rocks 
on  either  side  of  the  fissure,  the  amount  of  baking  being 
in  proportion  to  the  width  of  the  dike,  and  thus  to  the 
amount  of  heat  which  it  could  s:ive  forth.     A  dike  six 


306  OUTLINES  OF  THE  EARTH'S  HISTORY. 

inches  in  diameter  will  sometimes  barely  sear  its  walls, 
while  one  a  hundred  feet  in  width  will  often  alter  the 
strata  for  a  great  distance  on  either  side.  In  some  in- 
stances, as  in  the  coal  beds  near  Eichmond,  Va.,  dikes  oc- 
casionally cut  through  beds  of  bituminous  coal.  In  these 
cases  we  find  that  the  coal  has  been  converted  into  coke 
for  many  feet  either  side  of  a  considerable  injection.  The 
fact  that  the  dike  material  was  molten  is  still  further 
shown  by  the  occurrence  in  it  of  fragments  which  it  has 
taken  up  from  the  walls,  and  which  may  have  been  partly 
melted,  and  in  most  cases  have  clearly  been  much  heated. 

Where  dikes  extend  up  through  stratified  beds  which 
are  separated  from  each  other  by  distinct  layers,  along 
which  the  rock  is  not  firmly  bound  together,  it  now  and 
then  happens,  as  noted  by  Mr.  G.  K.  Gilbert,  of  the  United 
States  Geological  Survey,  that  the  lava  has  forced  its  way 
horizontally  between  these  layers,  gradually  uplifting  the 
overlying  mass,  which  it  did  not  break  through,  into  a 
dome-shaped  elevation.  These  side  flows  from  dikes  are 
termed  laccolites,  a  word  wliicli  signifies  the  pool-like  na- 
ture of  the  stony  mass  which  they  form  between  the 
strata. 

In  many  regions,  where  the  earth  has  worn  down  so 
as  to  reveal  the  zone  of  dikes  which  was  formed  at  a  great 
depth,  the  surface  of  the  country  is  fairly  laced  with  these 
intrusions.  Thus  on  Cape  Ann,  a  rocky  isle  on  the  east 
coast  of  Massachusetts,  having  an  area  of  about  twenty 
square  miles,  the  writer,  with  the  assistance  of  his  col- 
league, Prof.  E.  S.  Tarr,  found  about  four  hundred  dis- 
tinct dikes  exhibited  on  the  shore  line  where  the  rocks 
had  been  swept  bare  by  the  waves.  If  the  census  of  these 
intrusions  could  have  been  extended  over  the  whole  island, 
it  would  probably  have  appeared  that  the  total  number 
exceeded  five  thousand.  In  other  regions  square  miles  can 
be  found  where  the  dikes  intercepted  by  the  surface  occupy 
an  aggregate  area  greater  than  that  of  the  rocks  into 
which  they  have  been  intruded. 


THE   WORK  OF   UNDERGROUND  WATER.         307 

Now  and  then,  but  rarely,  the  student  of  dikes  finds 
one  where  the  bordering  walls,  in  place  of  having  the 
clean-cut  appearance  which  they  usually  exhibit,  has  its 
sides  greatly  worn  away  and  much  melted,  as  if  by  the 
long-continued  passage  of  the  igneous  fluid  through  the 
crevice.  Such  dikes  are  usually  very  wide,  and  are  prob- 
ably the  paths  through  which  lavas  found  their  way  to  the 
surface  of  the  earth,  pouring  forth  in  a  volcanic  eruption. 
In  some  cases  we  can  trace  their  relation  to  ancient  vol- 
canic cones  which  have  worn  down  in  all  their  part  which 
were  made  up  of  incoherent  materials,  so  that  there  re- 
mains only  the  central  pipe,  which  has  been  preserved 
from  decay  by  the  coherent  character  of  the  lava  which 
filled  it. 

The  hypothesis  that  dikes  are  driven  upward  into 
strata  by  the  pressure  of  the  beds  which  overlie  materials 
hot  and  soft  enough  to  be  put  in  motion  when  a  fissure 
enters  them,  and  that  their  movement  upward  through 
the  crevice  is  accounted  for  by  this  pressure,  makes  cer- 
tain features  of  these  intrusions  comprehensible.  Seeing 
that  very  long,  slender  dikes  are  found  penetrating  the 
rock,  which  could  not  have  had  a  high  temperature,  it 
becomes  difficult  to  understand  how  the  lava  could  have 
maintained  its  fluidity;  but  on  the  supposition  that  it  was 
impelled  forward  by  a  strong  pressure,  and  that  the  energy 
thus  transmitted  through  it  was  converted  into  heat,  we 
discover  a  means  whereby  it  could  have  been  retained  in 
the  liquid  condition,  even  when  forced  for  long  distances 
through  very  narrow  channels.  Moreover,  this  explanation 
accounts  for  the  fact  which  has  long  remained  unexplained 
that  dikes,  except  those  formed  about  volcanic  craters, 
rarely,  if  ever,  rise  to  the  surface. 

The  materials  contained  in  dikes  differ  exceedingly  in 
their  chemical  and  mineral  character.  These  variations 
are  due  to  the  differences  in  Nature  of  the  deposits  whence 
they  come,  and  also  in  a  measure  to  exchanges  which  take 
place  between  their  own  substance  and  that  of  the  rocks 


308  OUTLINES  OF  THE  EARTH'S  HISTORY. 

between  which  they  are  deposited.  This  process  often 
has  importance  of  an  economic  kind,  for  it  not  infre- 
quently leads  to  the  formation  of  metalliferous  veins  or 
other  aggregations  of  ores,  either  in  the  dike  itself  or  in 
the  country  rock.  The  way  in  which  this  is  brought  about 
may  be  easily  understood  by  a  familiar  example.  If  flesh 
be  placed  in  water  which  has  the  same  temperature,  no 
exchange  of  materials  will  take  place;  but  if  the  water 
be  heated,  a  circulation  will  be  set  up,  which  in  time  will 
bring  a  large  part  of  the  soluble  matter  into  the  surround- 
ing water.  This  movement  is  primarily  dependent  on  dif- 
ferences of  temperature,  and  consequently  differences  in 
the  quantity  of  soluble  substances  which  the  water  seeks 
to  take  up.  When  a  dike  is  injected  into  cooler  rocks,  such 
a  slow  circulation  is  induced.  The  water  contained  in  the 
interstices  of  the  stone  becomes  charged  with  mineral  ma- 
terials, if  such  exist  in  positions  where  it  can  obtain  pos- 
session of  them,  and  as  cooling  goes  on,  these  dissolved 
materials  are  deposited  in  the  manner  of  veins.  These 
veins  are  generally  laid  down  on  the  planes  of  contact 
between  the  two  kinds  of  stone,  but  they  may  be  formed 
in  any  other  cavities  which  exist  in  the  neighbourhood. 
The  formation  of  such  veins  is  often  aided  by  the  consid- 
erable shrinkage  of  the  lava  in  the  dike,  which,  when  it 
cools,  tends  to  lose  about  fifteen  per  cent  of  its  volume, 
and  is  thus  likely  to  leave  a  crevice  next  the  boundary 
walls.  Ores  thus  formed  afford  some  of  the  commonest 
and  often  the  richest  mineral  deposits.  At  Leadville,  in 
Colorado,  the  great  silver-bearing  lodes  probably  were 
produced  in  this  manner,  wherein  lavas,  either  those  of 
dikes  or  those  which  flowed  in  the  open  air,  have  come  in 
contact  with  limestones.  The  mineral  materials  originally 
in  the  once  molten  rock  or  in  the  limy  beds  was,  we  be- 
lieve, laid  down  on  ancient  sea  floors  in  the  remains  of 
organic  forms,  which  for  their  particular  uses  took  the 
materials  from  the  old  sea  water.  The  vein-making  action 
has  served  to  assemble  these  scattered  bits  of  metal  into 


THE  WORK  OF  UNDERGROUND  WATER.    309 

the  aggregation  which  constitutes  a  workable  deposit.  In 
time,  as  the  rocks  wear  down,  the  materials  of  the  veins 
are  again  taken  into  solution  and  returned  to  the  sea, 
thence  perhaps  to  tread  again  the  cycle  of  change. 

In  certain  dikes,  and  sometimes  also,  perhaps,  in  lavas 
known  as  basalts,  which  have  flowed  on  the  surface,  the 
rock  when  cooling,  from  the  shrinkage  which  then  occurs, 
has  broken  in  a  very  regular  way,  forming  hexagonal  col- 
umns which  are  more  or  less  divided  on  their  length  by 
joints.  When  worn  away  by  the  agencies  of  decay,  espe- 
cially where  the  material  forms  steep  cliffs,  a  highly  arti- 
ficial effect  is  produced,  which  is  often  compared,  where 
cut  at  right  angles  to  the  columns,  to  pavements,  or,  where 
the  division  is  parallel  to  the  columns,  to  the  pipes  of  an 
organ. 

What  we  know  of  dikes  inclines  us  to  the  opinion  that 
as  a  whole  they  represent  movements  of  softened  rock 
where  the  motion-compelling  agent  is  not  mainly  the  ex- 
pansion of  the  contained  water  which  gives  rise  to  vol- 
canic ejection,  but  rather  in  large  part  due  to  the  weight 
of  superincumbent  strata  setting  in  motion  materials  which 
were  somewhat  softened,  and  which  tended  to  creep,  as  do 
the  clays  in  deep  coal  mines.  It  is  evident,  however;  it 
is,  moreover,  quite  natural,  that  dike  work  is  somewhat 
mingled  with  that  produced  by  the  volcanic  forces;  but 
while  the  line  between  the  two  actions  is  not  sharp,  the 
discrimination  is  important,  and  occurs  with  a  distinctness 
rather  unusual  on  the  boundary  line  between  two  adjacent 
fields  of  phenomena. 

We  have  now  to  consider  the  general  effects  of  the 
earth's  interior  heat  so  far  as  that  body  of  temperature 
tends  to  drive  materials  from  the  depths  of  the  earth  to 
the  surface.  This  group  of  influences  is  one  of  the  most 
important  w^hich  operates  on  our  sphere;  as  we  shall 
shortly  see,  without  such  action  the  earth  would  in  time 
become  an  unfit  theatre  for  the  development  of  organic 


310  OUTLINES  OF  THE  EARTH'S  HISTORY. 

life.  To  perceive  the  effect  of  these  movements,  we  must 
first  note  that  in  the  great  rock-constructing  realm  of  the 
seas  organic  life  is  constantly  extracting  from  the  water 
substances,  such  as  lime,  potash,  soda,  and  a  host  of  other 
substances  necessary  for  the  maintenance  of  high-grade 
organisms,  depositing  these  materials  in  the  growing  strata. 
Into  these  beds,  which  are  buried  as  fast  as  they  form, 
goes  not  only  these  earthy  materials,  but  a  great  store  of 
the  sea  water  as  well.  The  result  would  be  in  course  of 
time  a  complete  withdrawal  into  the  depths  of  the  earth 
of  those  substances  which  play  a  necessary  part  in  organic 
development.  The  earth  would  become  more  or  less  com- 
pletely waterless  on  its  surface,  and  the  rocks  exposed  to 
view  would  be  composed  mainly  of  silica,  the  material 
which  to  a  great  extent  resists  solution,  and  therefore 
avoids  the  dissolving  which  overtakes  most  other  kinds 
of  rocks.  Here  comes  in  the  machinery  of  the  hot  springs, 
the  dikes,  and  the  volcanoes.  These  agents,  operating 
under  the  influence  of  the  internal  heat  of  the  earth,  are 
constantly  engaged  in  bearing  the  earthy  matter,  particu- 
larly its  precious  more  solvent  parts,  back  to  the  surface. 
The  hot  springs  and  volcanoes  work  swiftly  and  directly, 
and  return  the  water,  the  carbon  dioxide,  and  a  host  of 
other  vaporizable  and  soluble  and  fusible  substances  to 
the  realm  of  solar  activity,  to  the  living  surface  zone  of 
the  earth.  The  dikes  operate  less  immediately,  but  in 
the  end  to  the  same  effect.  They  lift  their  materials  miles 
above  the  level  where  they  were  originally  laid,  probably 
from  a  zone  which  is  rarely  if  ever  exposed  to  view,  placing 
them  near  the  surface,  where  the  erosive  agents  can  readily 
find  access  to  them. 

Of  the  three  agents  which  serve  to  export  earth  mate- 
rials" from  its  depths,  volcanoes  are  doubtless  the  most  im- 
portant. They  send  forth  the  greater  part  of  the  water 
which  is  expelled  from  the  rocks.  Various  computations 
which  the  writer  has  made  indicate  that  an  ordinary  vol- 
cano, such  as  ^tna,  in  times  of  most  intense  explosion, 


THE  WORK  OF  UNDERGROUND  WATER.    311 

may  send  forth  in  the  form  of  steam  one  fourth  of  a 
cubic  mile  or  more  of  water  during  each  day  of  its  dis- 
charge, and  in  a  single  great  eruption  may  pour  forth 
several  times  this  quantity.  In  its  history  ^tna  has  prob- 
ably returned  to  the  atmosphere  some  hundred  cubic  miles 
of  water  which  but  for  the  process  would  have  remained 
permanently  locked  up  in  its  rock  prison. 

The  ejection  of  rock  material,  though  probably  on  the 
average  less  in  quantity  than  the  water  which  escapes,  is 
also  of  noteworthy  importance.  The  volcanoes  of  Java  and 
the  adjacent  isles  have,  during  the  last  hundred  and  twenty 
years,  delivered  to  the  seas  more  earth  material  than  has 
been  carried  into  those  basins  by  the  great  rivers.  If  we 
could  take  account  of  all  the  volcanic  ejections  which 
have  occurred  in  this  time,  we  should  doubtless  find  that 
the  sum  of  the  materials  thus  cast  forth  into  the  oceans 
was  several  times  as  great  as  that  which  was  delivered 
from  the  lands  by  all  the  superficial  agents  which  wear 
them  away.  Moreover,  while  the  material  from  the  land, 
except  the  small  part  which  is  in  a  state  of  complete  solu- 
tion, all  falls  close  to  the  shore,  the  volcanic  waste,  because 
of  its  fine  division  or  because  of  the  blebs  of  air  which  its 
masses  contain,  may  float  for  many  years  before  it  finds  its 
way  to  the  bottom,  it  may  be  at  the  antipodes  of  the  point 
at  which  it  came  from  the  earth.  While  thus  journeying 
through  the  sea  the  rock  matter  from  the  volcanoes  is  apt 
to  become  dissolved  in  water;  it  is,  indeed,  doubtful  if  any 
considerable  part  of  that  which  enters  the  ocean  goes  by 
gravitation  to  its  floor.  The  greater  portion  probably 
enters  the  state  of  solution  and  makes  its  way  thence 
through  the  bodies  of  plants  and  animals  again  into  the 
ponderable  state. 

If  an  observer  could  view  the  earth  from  the  surface 
of  the  moon,  he  would  probably  each  day  behold  one  of 
these  storms  which  the  volcanoes  send  forth.  In  the  fort- 
night of  darkness,  even  with  the  naked  eye,  it  would  prob- 
ably be  possible  to  discern  at  any  time  several  eruptions, 
21 


312         OUTLINES  OF  THE  EARTH'S  HISTORY. 

some  of  which  would  indicate  that  the  earth's  surface 
was  ravaged  by  great  catastrophes.  The  nearer  view  of 
these  actions  shows  us  that  although  locally  and  in  small 
measure  they  are  harmful  to  the  life  of  the  earth,  they  are 
in  a  large  way  beneficent. 


CHAPTER  VIII. 

THE    SOIL. 

The  frequent  mention  which  it  has  been  necessary  to 
make  of  soil  phenomena  in  the  preceding  chapters  shows 
how  intimately  this  feature  in  the  structure  of  the  earth 
is  blended  with  all  the  elements  of  its  physical  history. 
It  is  now  necessary  for  us  to  take  up  the  phenomena  of 
soils  in  a  consecutive  manner. 

The  study  of  any  considerable  river  basin  enables  us 
to  trace  the  more  important  steps  which  lead  to  the  de- 
structure  and  renovation  of  the  earth's  detrital  coating. 
In  such  an  interpretation  we  note  that  everywhere  the  rocks 
which  were  built  on  the  sea  bottom,  and  more  or  less  made 
over  in  the  great  laboratory  of  the  earth^s  interior,  are  at 
the  surface^  when  exposed  to  the  conditions  of  the  atmos- 
phere, in  process  of  being  taken  to  pieces  and  returned  to 
the  sea.  This  action  goes  on  everywhere;  every  drop  of 
rain  helps  it.  It  is  aided  by  frost,  or  even  by  the  changes 
of  expansion  and  contraction  which  occur  in  the  rocks 
from  variations  of  heat.  The  result  is  that,  except  where 
the  slopes  are  steep,  the  surface  is  quickly  covered  with  a 
layer  of  fragments,  all  of  which  are  in  the  process  of  decay, 
and  ready  to  afford  some  food  to  plants.  Even  where  the 
rock  appears  bare,  it  is  generally  covered  with  lichens, 
which,  adhering  to  it,  obtain  a  share  of  nutriment  from  the 
decayed  material  which  they  help  to  hold  on  the  slope. 
When  they  have  retained  a  thin  sheet  of  the  debris,  mosses 
and  small  flowering  plants  help  the  work  of  retaining  the 

313 


314  OUTLINES  OF  THE  EARTH'S  HISTORY. 

detritus.  Soon  the  strong-rooted  bushes  and  trees  win  a 
foothold,  and  by  sending  their  rootlets,  which  are  at  first 
small  but  rapidly  enlarge,  into  the  crevices,  they  hasten 
the  disruption  of  the  stones. 

If  the  construction  of  soil  goes  on  upon  a  steep  cliff, 
the  quantity  retained  on  the  slope  may  be  small,  but  at  the 
base  we  find  a  talus,  composed  of  the  fragments  not  held 
by  the  vegetation,  which  gradually  increases  as  the  cliff 
wears  down,  until  the  original  precipice  may  be  quite  ob- 
literated beneath  a  soil  slope.  At  first  this  process  is  rapid; 
it  becomes  gradually  slower  and  slower  as  the  talus  mounts 
up  the  cliff  and  as  the  cliff  loses  its  steepness,  until  finally 
a  gentle  slope  takes  the  place  of  the  steep. 

From  the  highest  points  in  any  river  valley  to  the  sea 
level  the  broken-up  rock,  which  we  term  soil,  is  in  process 
of  continuous  motion.  Everywhere  the  rain  water,  flowing 
over  the  surface  or  soaking  through  the  porous  mass,  is 
conveying  portions  of  the  material  which  is  taken  into 
solution  in  a  speedy  manner  to  the  sea.  Everywhere  the 
expansion  of  the  soil  in  freezing,  or  the  movements  im- 
posed on  it  by  the  growth  of  roots,  by  the  overturning  of 
trees,  or  by  the  innumerable  borings  and  burrowings  which 
animals  make  in  the  mass,  is  through  the  action  of  gravita- 
tion slowly  working  down  the  slope.  Every  little  disturb- 
ance of  the  grains  or  fragments  of  the  soil  which  lifts  them 
up  causes  them  when  they  fall  to  descend  a  little  way  far- 
ther toward  the  sea  level.  Working  toward  the  streams, 
the  materials  of  the  soil  are  in  time  delivered  to  those 
flowing  waters,  and  by  them  urged  speedily,  though  in 
most  cases  interruptedly,  toward  the  ocean. 

There  is  another  element  in  the  movement  of  the  soils 
which,  though  less  appreciable,  is  still  of  great  importance. 
The  agents  of  decay  which  produce  and  remove  the  de- 
tritus, the  chemical  changes  of  the  bed  rock,  and  the  me- 
chanical action  which  roots  apply  to  them,  along  with  the 
solutional  processes,  are  constantly  lowering  the  surface  of 
the  mass.    In  this  way  we  can  often  prove  that  a  soil  con- 


THE  SOIL.  315 

tinuously  existing  has  worked  downward  through  many 
thousand  feet  of  strata.  In  this  process  of  downgoing  the 
country  on  which  the  layer  rests  may  have  greatly  changed 
its  form,  but  the  deposit,  under  favourable  conditions,  may 
continue  to  retain  some  trace  of  the  materials  which  it  de- 
rived from  beds  which  have  long  since  disappeared,  their 
position  having  been  far  up  in  the  spaces  now  occupied  by 
the  air.  Where  the  slopes  are  steep  and  streams  abound, 
we  rarely  find  detritus  which  belonged  in  rock  more  than  a 
hundred  feet  above  the  present  surface  of  the  soil.  Where, 
however,  as  on  those  isolated  table-lands  or  buttes  which 
abound  in  certain  portions  of  the  Mississippi  Valle}^,  as  well 
as  in  many  other  countries,  we  find  a  patch  of  soil  lying 
on  a  nearly  level  surface,  which  for  geologic  ages  has  not 
felt  the  effect  of  streams,  we  may  discover,  commingled 
in  the  debris,  the  harder  wreckage  derived  from  the  decay 
of  a  thousand  feet  or  more  of  vanished  strata. 

When  we  consider  the  effect  of  organic  life  on  the 
processes  which  go  on  in  the  soil,  we  first  note  the  large 
fact  that  the  development  of  all  land  vegetation  depends 
upon  the  existence  of  this  detritus — in  a  word,  on  the  slow 
movement  of  the  decaying  rocky  matter  from  the  point 
where  it  is  disrupted  to  its  field  of  rest  in  the  depths  of  the 
sea.  The  plants  take  their  food  from  the  portion  of  this 
rocky  waste  which  is  brought  into  solution  by  the  waters 
which  penetrate  the  mass.  On  the  plants  the  animals  feed, 
and  so  this  vast  assemblage  of  organisms  is  maintained. 
Not  only  does  the  land  life  maintain  itself  on  the  soil,  and 
give  much  to  the  sea,  but  it  serves  in  various  ways  to  pro- 
tect this  detrital  coating  from  too  rapid  destruction,  and 
to  improve  its  quality.  To  see  the  nature  of  this  work 
we  should  visit  a  region  where  primeval  forests  still  lie 
upon  the  slopes  of  a  hilly  region.  In  the  body  of  such  a 
wood  we  find  next  the  surface  a  coating  of  decayed  vege- 
table matter,  made  up  of  the  falling  leaves,  bark,  branches, 
and  trunks  which  are  constantly  descending  to  the  earth. 
Ordinarily,  this  layer  is  a  foot  or  more  in  thickness;  at  the 


316  OUTLINES  OF  THE  EARTH'S  HISTORY. 

top  it  is  almost  altogether  composed  of  vegetable  matter; 
at  the  bottom  it  verges  into  the  true  soil.  An  important 
effect  of  this  decayed  vegetation  is  to  restrain  the  move- 
ment of  the  surface  water.  Even  in  the  heaviest  rains, 
provided  the  mass  be  not  frozen,  the  water  is  taken  into  it 
and  delivered  in  the  manner  of  springs  to  the  larger 
streams.  We  can  better  note  the  measure  of  this  effect  by- 
observing  the  difference  in  the  ground  covered  by  this 
primeval  forest  and  that  which  we  find  near  by  which  has 
been  converted  into  tilled  fields.  With  the  same  degree  of 
rapidity  in  the  flow,  the  distinct  stream  channels  on  the 
tilled  ground  are  likely  to  be  from  twenty  to  a  hundred 
times  in  length  what  they  are  on  the  forest  bed.  The  re- 
sult is  that 'while  the  brook  which  drains  the  forested  area 
maintains  a  tolerably  constant  flow  of  clean  water,  the 
other  from  the  tilled  ground  courses  only  in  times  of  heavy 
rain,  and  then  is  heavily  charged  with  mud.  In  the  virgin 
conditions  of  the  soil  the  downwear  is  very  slow;  in  its 
artificial  state  this  wearing  goes  on  so  rapidly  that  the  slop- 
ing fields  are  likely  to  be  worn  to  below  the  soil  level  in  a 
few  score  years. 

Not  only  does  the  natural  coating  of  vegetation,  such 
as  our  forests  impose  upon  the  country,  protect  the  soil 
from  washing  away,  but  the  roots  of  the  larger  plants  are 
continually  at  work  in  various  ways  to  increase  the  fertility 
and  depth  of  the  stratum.  In  the  form  of  slender  fibrils 
these  underground  branches  enter  the  joints  and  bed  planes 
of  the  rock,  and  there  growing  they  disrupt  the  materials, 
giving  them  a  larger  surface  on  which  decay  may  operate. 
These  bits,  at  first  of  considerable  size,  are  in  turn  broken 
up  by  the  same  action.  Wliere  the  underlying  rocks  afford 
nutritious  materials,  the  branches  of  our  tap-rooted  trees 
sometimes  find  their  way  ten  feet  or  more  below  the  base 
of  the  true  soil.  Not  only  do  they  thus  break  up  the  stones, 
but  the  nutrition  which  they  obtain  in  the  depths  is 
brought  up  and  deposited  in  the  parts  above  the  ground, 
as  well  as  in  the  roots  which  lie  in  the  true  soil,  so  that 


THE  SOIL.  317 

when  the  tree  dies  it  becomes  available  for  other  plants. 
Thus  in  the  forest  condition  of  a  country  the  amount  of 
rock  material  contributed  to  the  deposit  in  general  so  far 
exceeds  that  which  is  taken  away  to  the  rivers  by  the  under- 
ground water  as  to  insure  the  deepening  of  the  soil  bed  to 
the  point  where  only  the  strongest  roots — those  belong- 
ing to  our  tap-rooted  trees — can  penetrate  through  it  to 
the  bed  rocks. 

Almost  all  forests  are  from  time  to  time  visited  by 
winds  which  uproot  the  trees.  When  they  are  thus  rent 
from  the  earth,  the  underground  branches  often  form  a 
disk  containing  a  thick  tangle  of  stones  and  earth,  and 
having  a  diameter  of  ten  or  fifteen  feet.  The  writer  has 
frequently  observed  a  hundred  cubic  feet  of  Soil  matter, 
some  of  it  taken  from  the  depth  of  a  yard  or  more,  thus 
uplifted  into  the  air.  In  the  path  of  a  hurricane  or  tor- 
nado we  may  sometimes  find  thousands  of  acres  which  have 
been  subjected  to  this  rude  overturning — a  natural  plough- 
ing. As  the  roots  rot  away,  the  dehris  which  they  held 
falls  outside  of  the  pit,  thus  forming  a  little  hillock  along 
the  side  of  the  cavity.  After  a  time  the  thrusting  action  of 
other  roots  and  the  slow  motion  of  the  soil  down  the  slope 
restore  the  surface  from  its  hillocky  character  to  its  origi- 
nal smoothness;  but  in  many  cases  the  naturalist  who  has 
learned  to  discern  with  his  feet  may  note  these  irregulari- 
ties long  after  it  has  been  recovered  with  the  forest. 

Great  as  is  the  effect  of  plants  on  the  soil,  that  influ- 
ence is  almost  equalled  by  the  action  of  the  animals  which 
have  the  habit  of  entering  the  earth,  finding  there  a  tem- 
porary abiding  place.  The  number  of  these  ground  forms 
is  surprisingly  great.  It  includes,  indeed,  a  host  of  crea- 
tures which  are  efficient  agents  in  enriching  the  earth. 
The  specie  3  of  earthworms,  some  of  which  occupy  forested 
districts  as  well  as  the  fields,  have  the  habit  of  passing  the 
soil  material  through  their  bodies,  extracting  from  the 
mass  such  nutriment  as  it  may  contain.  In  this  manner 
the  particles  of  mineral  matter  become  pulverized,  and  in 


318  OUTLINES  OF  THE  EARTH'S  HISTORY. 

a  measure  affected  by  chemical  changes  in  the  bodies  of 
the  creatures,  and  are  thus  better  fitted  to  afford  plant 
food.  Sometimes  the  amount  of  the  earth  which  the  crea- 
tures take  in  in  moving  through  their  burrows  and  void 
upon  the  surface  is  sufficient  to  form  annually  a  layer  on 
the  surface  of  the  ground  having  a  depth  of  one  twentieth 
of  an  inch  or  more.  It  thus  may  well  happen  that  the 
soil  to  the  depth  of  two  or  three  feet  is  completely  over- 
turned in  the  course  of  a  few  hundred  years.  As  the  parti- 
cles which  the  creatures  devour  are  rather  small,  the  tend- 
ency is  to  accumulate  the  finer  portions  of  the  soil  near 
the  surface  of  the  earth,  where  by  solution  they  may  con- 
tribute to  the  needs  of  the  lowly  plants.  It  is  probably 
due  to  the  action  of  these  creatures  that  small  relics  of 
ancient  men,  such  as  stone  tools,  are  commonly  found 
buried  at  a  considerable  depth  beneath  the  earth,  and 
rarely  appear  upon  the  surface  except  where  it  has  been 
subjected  to  deep  ploughing  or  to  the  action  of  running 
streams. 

Along 'with  the  earthworms,  the  ants  labour  to  over- 
turn the  soil;  frequently  they  are  the  more  effective  of 
the  two  agents.  The  common  species,  though  they  make 
no  permanent  hillocks,  have  been  observed  by  the  writer 
to  lay  upon  the  surface  each  year  as  much  as  a  quarter  of 
an  inch  of  sand  and  other  fine  materials  which  they  have 
brought  up  from  a  considerable  depth.  In  many  regions, 
particularly  in  those  occupied  by  glacial  drift,  and  pebbly 
alluvium  along  the  rivers,  the  effect  of  this  action,  like 
that  of  earthworms,  is  to  bring  to  the  surface  the  finer 
materials,  leaving  the  coarser  pebbles  in  the  depths.  In 
this  way  they  have  changed  the  superficial  character  of  the 
soil  over  great  areas;  we  may  say,  indeed,  over  a  large  part 
of  the  earth,  and  this  in  a  way  which  fits  it  better  to  serve 
the  needs  of  the  wild  plants  as  well  as  the  uses  of  the 
farmer. 

Many  thousand  species  of  insects,  particularly  the 
larger  beetles,  have  the  habit  of  passing  their  larval  state 


THE  SOIL.  319 

in  the  under  earth.  Here  they  generally  excavate  bur- 
rows, and  thus  in  a  way  delve  the  soil.  As  many  of 
them  die  before  reaching  maturity,  their  store  of  organic 
matter  is  contributed  to  the  mass,  and  serves  to  nourish 
the  plants.  If  the  student  will  carefully  examine  a  sec- 
tion of  the  earth  either  in  its  natural  or  in  its  tilled  state, 
he  will  be  surprised  to  find  how  numerous  the  grubs  are. 
They  may  often  be  found  to  the  number  of  a  score  or 
more  of  each  cubic  foot  of  material.  Many  of  the  species 
which  develop  underground  come  from  eggs  which  have 
carefully  been  encased  in  organic  matter  before  their  de- 
position in  the  earth.  Thus  some  of  the  carrion  beetles 
are  in  the  habit  of  laying  their  eggs  in  the  bodies  of  dead 
birds  or  field  mice,  which  tliey  then  bury  to  the  depth  of 
some  inches  in  the  earth.  In  this  way  nearly  all  the  small 
birds  and  mammals  of  our  woods  disappear  from  view  in 
a  few  hours  after  they  are  dead.  Other  specips  make  balls 
from  the  dung  of  cattle  in  which  they  lay  their  eggs,  after- 
ward rolling  the  little  spheres,  it  may  be  for  hundreds  of 
feet,  to  the  chambers  in  the  soil  which  they  have  previously 
prepared.  In  this  way  a  great  deal  of  animal  matter  is 
introduced  into  the  earth,  and  contributes  to  its  fertility. 
Many  of  our  small  mammals  have  the  habit  of  making 
their  dwelling  places  in  the  soil.  Some  of  them,  such  as 
the  moles,  normally  abide  in  the  subterranean  realm  for 
all  their  lives.  Others  use  the  excavations  as  places  of 
retreat.  In  any  case,  these  excavations  serve  to  move  the 
particles  of  the  soil  about,  and  the  materials  which  the 
animals  drag  into  the  earth,  as  well  as  the  excrement  of 
the  creatures,  act  to  enrich  it.  This  habit  of  taking  food 
underground  is  not  limited  to  the  mammals;  it  is  common 
with  the  ants,  and  even  the  earthworms,  as  noted  by  Charles 
Darwin  in  his  wonderful  essay  on  these  creatures,  are  ac- 
customed to  drag  into  their  burrows  bits  of  grass  and  the 
slender  leaves  of  pines.  It  is  not  known  what  purpose 
they  attain  by  these  actions,  but  it  is  sufficiently  common 
somewhat  to  affect  the  conditions  of  the  soil. 


320  OUTLINES  OF  THE  EARTH'S  HISTORY. 

The  result  of  these  complicated  works  done  by  ani- 
mals and  plants  on  the  soil  is  that  the  material  to  a  con- 
siderable depth  are  constantly  being  supplied  with  organic 
matter,  which,  along  with  the  mineral  material,  consti- 
tutes that  part  of  the  earth  which  can  support  vegetation. 
Experiment  will  readily  show  that  neither  crushed  rock 
nor  pure  vegetable  mould  will  of  itself  serve  to  maintain 
any  but  the  lowliest  vegetation.  It  requires  that  the  two 
materials  be  mixed  in  order  that  the  earth  may  yield  food 
for  ordinary  plants,  particularly  for  those  which  are  of 
use  to  man,  as  crops.  On  this  account  all  the  processes 
above  noted  whereby  the  waste  of  plant  and  animal  life 
is  carried  below  the  surface  are  of  the  utmost 'importance 
in  the  creation  and  preservation  of  the  soil.  It  has  been 
found,  indeed,  in  almost  all  cases,  necessary  for  the  farmer 
to  maintain  the  fertility  of  his  fields  to  plough-in  quan- 
tities of  such  organic  waste.  By  so  doing  he  imitates  the 
work  which  is  effected  in  virgin  soil  by  natural  action. 
As  the  process  is  costly  in  time  and  material,  it  is  often 
neglected  or  imperfectly  done,  with  the  result  that  the 
fields  rapidly  diminish  in  fertility. 

The  way  in  which  the  buried  organic  matter  acts  upon 
the  soil  is  not  yet  thoroughly  understood.  In  part  it  ac- 
complishes the  results  by  the  materials  which  on  its  decay 
it  contributes  to  the  soil  in  a  state  in  which  they  may 
readily  be  dissolved  and  taken  up  by  the  roots  into  their 
sap;  in  part,  however,  it  is  believed  that  they  better  the 
conditions  by  affording  dwelling  places  for  a  host  of  lowly 
species,  such  as  the  forms  which  are  known  as  bacteria. 
The  organisms  probably  aid  in  the  decomposition  of  the 
mineral  matter,  and  in  the  conversion  of  nitrogen,  which 
abounds  in  the  air  or  the  soil,  into  nitrates  of  potash  and 
soda — substances  which  have  a  very  great  value  as  fer- 
tilizers. Some  effect  is  produced  by  the  decay  of  the  for- 
eign matter  brought  into  the  soil,  which  as  it  passes  away 
leaves  channels  through  which  the  soil  water  can  more 
readily  pass. 


THE  SOIL.  321 

By  far  the  most  general  and  important  effect  arising 
from  the  decay  of  organic  matter  in  the  earth  is  to  be 
found  in  the  carbon  dioxide  which  is  formed  as  the  oxygen 
of  the  air  combines  with  the  carbon  which  all  organic 
material  contains.  As  before  noted,  water  thus  charged 
has  its  capacity  for  taking  other  substances  into  solution 
vastly  increased,  and  on  this  solvent  action  depends  in 
large  part  the  decay  of  the  bed  rocks  and  the  solution  of 
materials  which  are  to  be  appropriated  by  the  plants. 

Having  now  sketched  the  general  conditions  which 
lead  to  the  formation  of  soils,  we  must  take  account  of 
certain  important  variations  in  their  conditions  due  to 
differences  in  the  ways  in  which  they  are  formed  and 
preserved.  These  matters  are  not  only  of  interest  to  the 
geologist,  but  are  of  the  utmost  importance  to  the  life  of 
mankind,  as  well  as  all  the  lower  creatures  which  dwell 
upon  the  lands.  First,  we  should  note  that  soils  are  di- 
visible into  three  great  groups,  which,  though  not  sharply 
parted  from  each  other,  are  sufficiently  peculiar  for  the 
purposes  of  classification.  Where  the  earth  material  has 
been  derived  from  the  rocks  which  nearly  or  immediately 
Underlie  it,  we  have  a  group  of  soils  which  may  be  entitled 
those  of  immediate  derivation — that  is,  derived  from  rocks 
near  by,  or  from  beds  which  once  overlaid  the  level  and 
have  since  been  decayed  away.  Next,  we  have  alluvial 
soils,  those  composed  of  materials  which  have  been  trans- 
ported by  streams,  commonly  from  a  great  distance,  and 
laid  down  on  their  flood  plains.  Third,  the  soils  the  min- 
eral matters  of  which  have  been  brought  into  their  posi- 
tion by  the  action  of  glaciers;  these  in  a  way  resemble 
those  formed  by  rivers,  but  the  materials  are  generally 
imperfectly  sorted,  coarse  and  fine  being  mingled  together. 
Last  of  all,  we  have  the  soils  due  to  the  accumulation  of 
blown  dust  or  blown  sand,  which,  unlike  the  others,  oc- 
cupy but  a  small  part  of  the  land  surface.  It  would  be 
possible,  indeed,  to  make  yet  another  division,  including 
those  areas  which  when  emerging  from  the  sea  were  cov- 


322  OUTLINES  OF  THE  EARTH'S  HISTORY. 

ered  with  fine,  uncemented  detritus  ready  at  once  to  serve 
the  purposes  of  a  soil.  Only  here  and  there,  and  but  sel- 
dom, do  we  find  soils  of  this  nature. 

It  is  characteristic  of  soils  belonging  to  the  group  to 
which  we  have  given  the  title  of  immediate  derivation  that 
they  have  accumulated  slowly,  that  they  move  very  grad- 
ually down  the  slopes  on  which  they  lie,  and  that  in  all 
cases  they  represent,  with  a  part  of  their  mass  at  least, 
levels  of  rock  which  have  disappeared  from  the  region 
which  they  occupied.  The  additions  made  to  their  mass 
are  from  below,  and  that  mass  is  constantly  shrinking, 
generally  at  a  pretty  rapid  rate,  by  the  mineral  matter 
which  is  dissolved  and  goes  away  with  the  spring  water. 
They  also  are  characteristically  thin  on  steep  slopes,  thick- 
ening toward  the  base  of  the  incline,  where  the  diminished 
grade  permits  the  soil  to  move  slowly,  and  therefore  to 
accumulate. 

In  alluvial  soils  we  find  accumulations  which  are  char- 
acterized by  growth  on  their  upper  surfaces,  and  by  the 
distant  transportation  of  the  materials  of  which  they  are 
composed.  In  these  deposits  the  outleaching  removes  vast 
amounts  of  the  materials,  but  so  long  as  the  floods  from 
time  to  time  visit  their  surfaces  the  growth  of  the  deposits 
is  continued.  This  growth  rarely  takes  place  from  the 
waste  of  the  bed  rocks  on  which  the  alluvium  lies.  It  is 
characteristic  of  alluvial  soils  that  they  are  generally  made 
up  of  debris  derived  from  fields  where  the  materials  have 
undergone  the  change  which  we  have  noted  in  the  last 
paragraph;  therefore  these  latter  deposits  have  through- 
out the  character  which  renders  the  mineral  materials 
easily  dissolved.  Moreover,  the  mass  as  it  is  constructed 
is  commonly  mingled  with  a  great  deal  of  organic  waste, 
which  serves  to  promote  its  fertility.  On  these  accounts 
alluvial  grounds,  though  they  vary  considerably  in  fer- 
tility, commonly  afford  the  most  fruitful  fields  of  any 
region.  They  have,  moreover,  the  signal  advantage  that 
they  often  may  be  refreshed  by  allowing  the  flood  waters 


THE  SOIL.  323 

to  visit  them,  an  action  which  but  for  the  interference  of 
man  commonly  takes  place  once  each  year.  Thus  in  the 
valley  of  the  Nile  there  are  fields  which  have  been  giving 
rich  grain  harvests  probably  for  more  than  four  thousand 
years,  without  any  other  effective  fertilizing  than  that  de- 
rived from  the  mud  of  the  great  river. 

The  group  of  glaciated  soils  differs  in  many  ways  from 
either  of  those  mentioned.  In  it  we  find  the  mineral  mat- 
ter to  have  been  broken  up,  transported,  and  accumulated 
without  the  influence  of  those  conditions  which  ordinarily 
serve  to  mix  rock  debris  with  organic  matter  during  the 
process  by  which  it  is  broken  into  bits.  When  vegetation 
came  to  preoccupy  the  fields  made  desolate  by  glacial 
action,  it  found  in  most  places  more  than  sufficient  ma- 
terial to  form  soils,  but  the  greater  part  of  the  matter  was 
in  the  condition  of  pebbles  of  very  hard  rock  and  sand 
grains,  fragments  of  silex.  Fortunately,  the  broken-up 
state  of  this  material,  by  exposing  a  great  surface  of  the 
rocky  matter  to  decay,  has  enabled  the  plants  to  convert 
a  portion  of  the  mass  into  earth  fit  for  the  uses  of  their 
roots.  But  as  the  time  which  has  elapsed  since  the  dis- 
appearance of  the  glaciers  is  much  less  than  that  occupied 
in  the  formation  of  ordinary  soil,  this  decay  has  in  most 
cases  not  yet  gone  very  far,  so  that  in  a  cubic  foot  of  gla- 
ciated waste  the  amount  of  material  available  for  plants 
is  often  only  a  fraction  of  that  held  in  the  soils  of  imme- 
diate derivation. 

In  the  greater  portion  of  the  fields  occupied  by  glacial 
waste  the  processes  which  lead  to  the  introduction  of 
organic  matter  into  the  earth  have  not  gone  far  enough 
to  set  in  effective  work  the  great  laboratory  which  has  to 
operate  in  order  to  give  fertile  soil.  The  pebbles  hinder 
the  penetration  of  the  roots  as  well  as  the  movement  of 
insects  and  other  animals.  There  has  not  been  time 
enough  for  the  overturning  of  trees  to  bring  about  a  cer- 
tain admixture  of  vegetable  matter  with  the  soil — in  a 
word,  the  process  of  soil-making,  though  the  first  condi- 


321  OUTLINES  OF  THE  EARTH'S  HISTORY. 

tion,  that  of  broken-up  rock,  has  been  accomplished,  is  as 
yet  very  incomj)lete.  It  needs,  indeed,  care  in  the  intro- 
duction of  organic  matter  for  its  completion. 

It  is  characteristic  of  glacial  soils  that  they  are  indefi- 
nitely deep.  This  often  is  a  disadvantageous  feature,  for 
the  reason  that  the  soil  water  may  pass  so  far  down  into 
the  earth  that  the  roots  are  often  deprived  of  the  moisture 
which  they  need,  and  which  in  ordinary  soils  is  retained 
near  the  surface  by  the  hard  underlayer.  On  the  other 
hand,  where  the  glacial  waste  is  made  up  of  pebbles 
formed  from  rocks  of  varied  chemical  composition,  which 
contain  a  considerable  share  of  lime,  potash,  soda,  and 
other  substances  which  are  required  by  plants,  the  very 
large  surface  which  they  expose  to  decay  provides  the  soil 
with  a  continuous  enrichment.  In  a  cubic  foot  of  pebbly 
glacial  earth  we  often  find  that  the  mass  offers  several  hun- 
dred times  as  much  surface  to  the  action  of  decay  as  is 
afforded  by  the  underlying  solid  bed  rock  from  which  a 
soil  of  immediate  derivation  has  to  win  its  mineral  sup- 
ply. Where  the  pebbly  glacial  waste  is  provided  with 
a  mixture  of  vegetable  matter,  the  process  of  decay  com- 
monly goes  forward  with  considerable  rapidity.  If  the 
supply  of  such  matter  is  large,  such  as  may  be  produced 
by  ploughing  in  barnyard  manure  or  green  crops,  the  nu- 
tritive value  of  the  earth  may  be  brought  to  a  very  high 
point. 

It  is  a  familiar  experience  in  regions  where  glacial 
soils  exist  that  the  earth  beneath  the  swamps  when  drained 
is  found  to  be  extraordinarily  well  suited  for  farming  pur- 
poses. On  inspecting  the  pebbles  from  such  places,  we 
observe  that  they  are  remarkably  decayed.  Where  the 
masses  contain  large  quantities  of  feldspar,  as  is  the  case 
in  the  greater  part  of  our  granitic  and  other  crystalline 
rocks,  this  material  in  its  decomposition  is  converted  into 
kaolin  or  feldspar  clay,  and  gives  the  stones  a  peculiar 
white  appearance,  which  marks  the  decomposition,  and 
indicate?  the  process  by  which  a  great  variety  of  valuable 


THE  SOIL.  325 

soil  ingredients  are  brought  into  a  state  where  they  may 
be  available  for  plants. 

In  certain  parts  of  the  glacial  areas,  particularly  in 
the  region  near  the  margin  of  the  ice  sheet,  where  the 
glacier  remained  in  one  position  for  a  considerable  time, 
we  find  extensive  deposits  of  silicious  sand,  formed  of  the 
materials  which  settled  from  the  under-ice  stream,  near 
where  they  escaped  from  the  glacial  cavern.  These  kames 
and  sand  plains,  because  of  the  silicious  nature  of  their 
materials  and  the  very  porous  nature  of  the  soil  which 
they  afford,  are  commonly  sterile,  or  at  most  render  a 
profit  to  the  tiller  by  dint  of  exceeding  care.  Thus  in 
Massachusetts,  although  the  first  settlers  seized  upon  these 
grounds,  and  planted  their  villages  upon  them  because 
the  forests  there  were  scanty  and  the  ground  free  from 
encumbering  boulders,  were  soon  driven  to  betake  them- 
selves to  those  areas  where  the  drift  was  less  silicious,  and 
where  the  pebbles  afforded  a  share  of  clay.  Very  extensive 
fields  of  this  sandy  nature  in  southeastern  New  England 
have  never  been  brought  under  tillage.  Thus  on  the  island 
of  Martha's  Vineyard  there  is  a  connected  area  containing 
about  thirty  thousand  acres  which  lies  in  a  very  favourable 
position  for  tillage,  but  has  been  found  substantially 
worthless  for  such  use.  The  farmers  have  found  it  more 
advantageous  to  clear  away  the  boulders  from  the  coarser 
drift  in  order  to  win  soil  which  would  give  them  fair  re- 
turns. 

Those  areas  which  are  occupied  by  soil  materials  which 
have  been  brought  into  their  position  by  the  action  of  the 
wind  may,  as  regards  their  character,  be  divided  into  two 
very  distinct  groups — the  dunes  and  loess  deposits.  In 
the  former  group,  where,  as  we  have  noted  (see  page  123), 
the  coarse  sea  sands  or  those  from  the  shores  of  lakes  are 
driven  forward  as  a  marching  hillock,  the  grains  of  the 
material  are  almost  always  silicious.  The  fragments  in 
the  motion  are  not  taken  up  into  the  air,  but  are  blown 
along  the  surface.     Such  dune  accumulations  afford  an 


326  OUTLINES  OF  THE  EARTH'S  HISTORY. 

earth  which  is  even  more  sterile  than  that  of  the  glacial 
sand  plains,  where  there  is  generally  a  certain  admixture 
of  pebbles  from  rocks  which  by  their  decomposition  may 
afford  some  elements  of  fertility.  Fortunately  for  the  in- 
terests of  man,  these  wind-borne  sands  occupy  but  a  small 
area;  in  North  America,  in  the  aggregate,  there  probably 
are  not  more  than  one  thousand  square  miles  of  such  de- 
posits. 

Where  the  rock  material  drifted  by  the  winds  is  so  fine 
that  it  may  rise  into  the  air  in  the  form  of  dust,  the  ac- 
cumulations made  of  it  generally  afford  a  fertile  soil,  and 
this  for  the  reason  that  they  are  composed  of  various 
kinds  of  rock,  and  not,  as  in  the  case  of  dunes,  of  nearly 
pure  silica.  In  some  very  rare  cases,  where  the  seashore 
is  bordered  by  coral  reefs,  as  it  is  in  parts  of  southern 
Florida,  and  the  strand  is  made  up  of  limestone  bits  de- 
rived from  the  hard  parts  which  the  polyps  secrete,  small 
dunes  are  made  of  limy  material.  Owing,  however,  in 
part  to  the  relatively  heavy  nature  of  this  substance,  as 
well  as  to  the  rapid  manner  in  which  its  grains  become  ce- 
mented together,  such  limestone  dunes  never  attain  great 
size  nor  travel  any  distance  from  their  point  of  origin. 

As  before  noted,  dust  accumulations  form  the  soil  in 
extended  areas  which  lie  to  the  leeward  of  great  deserts. 
Thus  a  considerable  part  of  western  China  and  much  of 
the  United  States  to  the  west  of  the  Mississippi  is  covered 
by  these  wind-blown  earths.  Wherever  the  rainfall  is  con- 
siderable these  loess  deposits  have  proved  to  have  a  high 
agricultural  value. 

Where  a  region  has  an  earth  which  has  recently  passed 
from  beneath  the  sea  or  a  great  lake,  the  surface  is  com- 
monly covered  by  incoherent  detritus  which  has  escaped 
consolidation  into  hard  rock  by  the  fact  that  it  has  not 
been  buried  and  thus  brought  into  the  laboratory  of  the 
earth's  crust.  When  such  a  region  becomes  dry  land,  the 
materials  are  immediately  ready  to  enter  into  the  state 
of  soil.     They  commonly  contain  a  good  deal  of  waste 


THE  SOIL.  327 

derived  from  the  organic  life  which  dwelt  upon  the  sea 
bottom  and  was  embedded  in  the  strata  as  they  were 
formed.  Where  these  accumulations  are  made  in  a  lake, 
the  land  vegetation  at  once  possesses  the  tield,  even  a  single 
year  being  sufficient  for  it  to  effect  its  establishment. 
Where  the  lands  emerge  from  the  sea,  it  requires  a  few 
years  for  the  salt  water  to  drain  away  so  that  the  earth  can 
be  fit  for  the  uses  of  plants.  In  a  general  way  these  sea- 
bottom  soils  resemble  those  formed  in  the  alluvial  plains. 
They  are,  however,  commonly  more  sandy,  and  their  sub- 
stances less  penetrated  by  that  decay  which  goes  on  very 
freely  in  the  atmosphere  because  of  the  abundant  supply 
of  oxygen,  and  but  slowly  on  the  sea  floor.  Moreover, 
the  marine  deposits  are  generally  made  up  in  large  part 
of  silicious  sand,  a  material  which  is  produced  in  large 
quantities  by  the  disruption  of  the  rocks  along  the  sea 
coast.  The  largest  single  field  of  these  ocean-bottom  soils 
of  North  America  is  found  in  the  lowland  region  of  the 
southern  United  States,  a  wide  belt  of  country  extending 
along  the  coast  from  the  Rio  Grande  to  New  York.  Al- 
though the  streams  have  channelled  shallow  valleys  in 
the  beds  of  this  region,  the  larger  part  of  its  surface  still 
has  the  peculiar  features  of  form  and  composition  which 
were  impressed  upon  it  when  it  lay  below  the  surface  of 
the  sea. 

Local  variations  in  the  character  of  the  soil  covering 
are  exceedingly  numerous,  and  these  differences  of  condi- 
tion profoundly  affect  the  estate  of  man.  We  shall  there- 
fore consider  some  of  the  more  important  of  these  condi- 
tions, with  special  reference  to  their  origin. 

The  most  important  and  distinctly  marked  variation 
ip  the  fertility  of  soils  is  that  which  is  produced  by  dif- 
ferences in  the  rainfall.  No  parts  of  the  earth  are  en- 
tirely lacking  in  rain,  but  over  considerable  areas  the  pre- 
cipitation does  not  exceed  half  a  foot  a  year.  In  such 
realms  the  soil  is  sterile,  and  the  natural  coating  of  vege- 
tation limited  to  those  plants  which  can  subsist  on  dew 
22 


328  OUTLINES  OF  THE  EARTH'S  HISTORY. 

or  which  can  take  on  an  occasional  growth  at  such  times 
as  moisture  may  come  upon  them.  With  a  slight  increase 
in  precipitation,  the  soil  rapidly  increases  in  productivity, 
so  that  we  may  say  that  where  as  much  as  about  ten  inches 
of  water  enters  the  earth  during  the  summer  half  of  the 
year,  it  becomes  in  a  considerable  measure  fit  for  agri- 
culture. Observations  indicate  that  the  conditions  of  fer- 
tility are  not  satisfied  where  the  rainfall  is  just  sufficient 
to  fill  the  pores  of  the  soil;  there  must  be  enough  water 
entering  the  earth  to  bring  about  a  certain  amount  of 
outflow  in  the  form  of  springs.  The  reason  of  this  need 
becomes  apparent  when  we  study  the  evident  features  of 
those  soils  which,  though  from  season  to  season  charged 
with  water,  do  not  yield  springs,  but  send  the  moisture 
away  through  the  atmosphere.  Wherever  these  conditions 
occur  we  observe  that  the  soil  in  dry  seasons  becomes 
coated  with  a  deposit  of  mineral  matter,  which,  because 
of  its  taste,  has  received  the  name  of  alkali.  The  origin 
of  this  coating  is  as  follows:  The  pores  of  the  soil,  charged 
from  year  to  year  with  sufficient  water  to  fill  them,  be- 
come stored  with  a  fluid  which  contains  a  very  large 
amount  of  dissolved  mineral  matter — too  much,  indeed, 
to  permit  the  roots  of  plants,  save  a  few  species  which 
have  become  accustomed  to  the  conditions,  to  do  their 
appointed  work.  In  fact,  this  water  is  much  like  that  of 
the  sea,  which  the  roots  of  only  a  few  of  our  higher  plants 
can  tolerate.  When  the  dry  season  comes  on,  the  heat 
of  the  sun  evaporates  the  water  at  the  surface,  leaving 
behind  a  coating  composed  of  the  substances  which  the 
water  contains.  The  soil  below  acts  in  the  manner  of  a 
lamp-wick  to  draw  up  fluid  as  rapidly  as  the  heat  burns 
it  away.  When  the  soil  water  is  as  far  as  possible  ex- 
hausted, the  alkali  coating  may  represent  a  considerable 
part  of  the  soluble  matter  of  the  soil,  and  in  the  next 
rainy  season  it  may  return  in  whole  or  in  part  to  the  under- 
earth,  again  to  be  drawn  in  the  manner  before  described  to 
the  upper  level.    It  is  therefore  only  when  a  considerable 


THE  SOIL.  329 

share  of  the  ground  water  goes  forth  to  the  streams  in 
each  year  that  the  alkaline  materials  are  in  quantity  kept 
down  to  the  point  where  the  roots  of  our  crop-giving  plants 
can  make  due  use  of  the  soil.  Where,  in  an  arid  region, 
the  ground  can  be  watered  from  the  enduring  streams  or 
from  artificial  reservoirs,  the  main  advantage  arising  from 
the  process  is  commonly  found  in  the  control  which  it 
gives  the  farmer  in  the  amount  of  the  soil  water.  He  can 
add  to  the  rainfall  sufficient  to  take  away  the  excess  of 
mineral  matter.  When  such  soils  are  first  brought  under 
tillage  it  is  necessary  to  use  a  large  amount  of  water  from 
the  canals,  in  order  to  wash  away  the  old  store  of  alkali. 
After  that  a  comparatively  small  contribution  will  often 
keep  the  soil  in  excellent  condition  for  agriculture.  It 
has  been  found,  however,  in  the  irrigated  lands  beside 
the  Nile  that  where  too  much  saving  is  practised  in  the 
irrigation,  the  alkaline  coating  will  appear  where  it  has 
been  unknown  before,  and  with  it  an  unfitness  of  the  earth 
to  bear  crops. 

Although  the  crust  of  mineral  matters  formed  in  the 
manner  above  described  is  characteristic  of  arid  countries, 
and  in  general  peculiar  to  them,  a  similar  deposit  may 
under  peculiar  conditions  be  formed  in  regions  of  great 
rainfall.  Thus  on  the  eastern  coast  of  New  England,  where 
the  tidal  marshes  have  here  and  there  been  diked  from 
the  sea  and  brought  under  tillage,  the  dissolved  mineral 
matters  of  the  soil,  which  are  excessive  in  quantity,  are 
drawn  to  the  surface,  forming  a  coating  essentially  like 
that  which  is  so  common  in  arid  regions.  The  writer  has 
observed  this  crust  on  such  diked  lands,  having  a  thick- 
ness of  an  eighth  of  an  inch.  In  fact,  this  alkali  coating 
represents  merely  the  extreme  operation  of  a  process  which 
is  going  on  in  all  soils,  and  which  contributes  much  to 
their  fertility.  When  rain  falls  and  passes  downward 
into  the  earth,  it  conveys  the  soluble  matter  to  a  depth 
below  the  surface,  often  to  beyond  the  point  where  our 
ordinary  crop  plants,  such  as  the  small  grains,  can  have 


330         OUTLINES  OF  THE  EARTH'S  HISTORY. 

access  to  it,  and  this  for  the  reason  that  their  roots  do 
not  penetrate  deeply.  When  dry  weather  comes  and  evap- 
oration takes  place  from  the  surface,  the  fluid  is  drawn 
up  to  the  upper  soil  layer,  and  there,  in  process  of  evap- 
oration, deposits  the  dissolved  materials  which  it  con- 
tains. Thus  the  mineral  matter  which  is  fit  for  plant 
food  is  constantly  set  in  motion,  and  in  its  movement 
passes  the  rootlets  of  the  plants.  It  is  probably  on  this 
account — at  least  in  part — that  very  wet  weather  is  almost 
as  unfavourable  to  the  farmer  as  exceedingly  dry,  the 
normal  alternation  in  the  conditions  being,  as  is  well 
known,  best  suited  to  his  needs. 

So  long  as  the  earth  is  subjected  to  conditions  in 
which  the  rainfall  may  bring  about  a  variable  amount  of 
water  in  the  superficial  detrital  layer,  we  find  normal 
fruitful  soils,  though  in  their  more  arid  conditions  they 
may  be  fit  for  but  few  species  of  plants.  When,  by  in- 
creasing aridity,  we  pass  to  conditions  where  there  is  no 
tolerably  permanent  store  of  water  in  the  debris^  the  ma- 
terial ceases  to  have  the  qualities  of  a  soil,  and  becomes 
mere  rock  waste.  At  the  other  extreme  of  the  scale  we 
pass  to  conditions  where  the  water  is  steadfastly  main- 
tained in  the  interstices  of  the  detritus,  and  there  again 
the  characteristic  of  the  soil  and  its  fitness  for  the  uses  of 
land  vegetation  likewise  disappear.  In  a  word,  true  soil 
conditions  demand  the  presence  of  moisture,  but  that  in 
insufficient  quantities,  to  keep  the  pores  of  the  earth  con- 
tinually filled;  where  they  are  thus  filled,  we  have  the 
condition  of  swamps.  Between  these  extremes  the  level 
at  which  the  water  stands  in  the  soil  in  average  seasons 
is  continually  varying.  In  rainy  weather  it  may  rise 
quite  to  the  surface;  in  a  dry  season  it  may  sink  far  down. 
As  this  water  rises  and  falls,  it  not  only  moves,  as  before 
noted,  the  soluble  mineral  materials,  but  it  draws  the 
air  into  and  expels  it  from  the  earth  with  each  move- 
ment. This  atmospheric  circulation  of  the  soil,  as  has 
been  proved  by  experiment,  is  of  great  importance  in 


THE  SOIL.  331 

maintaining  its  fertility;  the  successive  charges  of  air 
supply  the  needs  of  the  microscopic  underground  crea- 
tures which  play  a  large  part  in  enriching  the  soil,  and 
the  direct  effect  of  the  oxygen  in  promoting  decay  is 
likewise  considerahle.  A  part  of  the  work  which  is  ac- 
complished by  overturning  the  earth  in  tillage  consists 
in  this  introduction  of  the  air  into  the  pores  of  the  soil, 
where  it  serves  to  advance  the  actions  which  bring  min- 
eral matters  into  solution. 

In  the  original  conditions  of  any  country  which  is 
the  seat  of  considerable  rainfall,  and  where  the  river 
system  is  not  so  far  developed  as  to  provide  channels 
for  the  ready  exit  of  the  waters,  we  commonly  find  very 
extensive  swamps;  these  conditions  of  bad  drainage  al- 
most invariably  exist  where  a  region  has  recently  been 
elevated  above  the  level  of  the  sea,  and  still  retains  the 
form  of  an  irregular  rolling  plain  common  to  sea  floors,  and 
also  in  regions  where  the  work  done  by  glaciers  has  con- 
fused the  drainage  which  the  antecedent  streams  may  have 
developed.  In  an  old,  well-elaborated  river  system  swamps 
are  commonly  absent,  or,  if  they  occur,  are  due  to  local 
accidents  of  an  unimportant  nature. 

For  our  purpose  swamps  may  be  divided  into  three 
groups — climbing  bogs,  lake  bogs,  and  marine  marshes. 
The  first  two  of  these  groups  depend  on  the  movements 
of  the  rain  water  over  the  land;  the  third  on  the  action 
of  the  tides.  Beginning  our  account  with  the  first  and 
most  exceptional  of  these  groups,  we  note  the  following 
features  in  their  interesting  history: 

Wherever  in  a  humid  region,  on  a  gentle  slope — say 
with  an  inclination  not  exceeding  ten  feet  to  the  mile — 
the  soil  is  possessed  by  any  species  of  plants  whose  stems 
grow  closely  together,  so  that  from  their  decayed  parts 
a  spongelike  mass  is  produced,  we  have  the  conditions 
which  favour  the  development  of  climbing  bogs.  Be- 
ginning usually  in  the  shores  of  a  pool,  these  plants,  neces- 
sarily of  a  water-loving  species,  retain  so  much  moisture 


332  OUTLINES  OF  THE  EARTH'S  HISTORY. 

in  the  spongy  mass  which  they  form  that  they  gradually 
extend  up  the  slope.  Thus  extending  the  margin  of  their 
field,  and  at  the  same  time  thickening  the  deposit  which 
they  form,  these  plants  may  build  a  climbing  bog  over 
the  surface  until  steeps  are  attained  where  the  inclina- 
tion is  so  great  that  the  necessary  amount  of  water  can 
not  be  held  in  the  spongy  mass,  or  where,  even  if  so  held, 
the  whole  coating  will  in  time  slip  down  in  the  manner 
of  an  avalanche. 

The  greater  part  of  the  climbing  bogs  of  the  world  are 
limited  to  the  moist  and  cool  regions  of  high  latitudes, 
where  species  of  moss  belonging  to  the  genus  Sphagnum 
plentifully  flourish.  These  plants  can  only  grow  where 
they  are  continuously  supplied  with  a  bath  of  water  about 
their  roots.  They  develop  in  lake  bogs  as  far  south  as 
Mexico,  but  in  the  climbing  form  they  are  hardly  trace- 
able south  of  New  England,  and  are  nowhere  extensively 
developed  within  the  limits  of  the  United  States.  In 
more  northern  parts  of  this  continent,  and  in  northwest- 
ern Europe,  particularly  in  the  moist  climate  of  Ireland, 
climbing  bogs  occupy  great  areas,  and  hold  up  their  lakes 
of  interstitially  contained  water  over  the  slopes  of  hills, 
where  the  surface  rises  at  the  rate  of  thirty  feet  or  more 
to  the  mile.  So  long  as  the  deposit  of  decayed  vegetable 
matter  which  has  accumulated  in  this  manner  is  thin, 
therefore  everywhere  penetrated  by  the  fibrous  roots  of  the 
moss,  it  may  continue  to  cling  to  its  sloping  bed;  but  when 
it  attains  a  considerable  thickness,  and  the  roots  in  the 
lower  part  decay,  the  pulpy  mass,  water-laden  in  some  time 
of  heavy  rain,  break  away  in  a  vast  torrent  of  thick,  black 
mud,  which  may  inundate  the  lower  lands,  causing  wide- 
spread destruction. 

In  more  southern  countries,  other  water-loving  plants 
lead  to  the  formation  of  climbing  bogs.  Of  these,  the 
commonest  and  most  effective  are  the  species  of  reeds,  of 
which  our  Indian  cane  is  a  familiar  example.  Brakes 
of  this  vegetation,  plentifully  mingled  with  other  species 


THE  SOIL.  333 

of  aquatic  growth,  form  those  remarkable  climbing  bogs 
known  as  the  Dismal  and  other  swamps,  which  numerously 
occur  along  the  coast  line  of  the  United  States  from 
southern  Maryland  to  eastern  Texas.  Climbing  bogs  are 
particularly  interesting,  not  only  from  the  fact  that  they 
are  eminently  peculiar  effects  of  plant  growth,  but  be- 
cause they  give  us  a  vivid  picture  of  those  ancient  morasses 
in  which  grew  the  plants  that  formed  the  beds  of  vege- 
table matter  now  appearing  in  the  state  of  coal.  Each 
such  bed  of  buried  swamp  material  was,  with  rare  excep- 
tions, where  the  accumulation  took  place  in  lakes,  gathered 
in  climbing  bogs  such  as  we  have  described. 

Lake  bogs  occur  in  all  parts  of  the  world,  but  in  their 
best  development  are  limited  to  relatively  high  latitudes, 
and  this  for  the  reason  that  the  plants  which  form  vege- 
table matter  grow  most  luxuriantly  in  cool  climates  and 
in  regions  where  the  level  of  the  basin  is  subject  to  less 
variation  than  occurs  in  the  alternating  wet  and  dry 
seasons  which  exist  in  nearly  all  tropical  regions.  The 
fittest  conditions  are  found  in  glaciated  regions,  where, 
as  before  noted,  small  lakes  are  usually  very  abundant. 
On  the  shores  of  one  of  these  pools,  of  size  not  so  great 
that  the  waves  may  attain  a  considerable  height,  or  in  the 
sheltered  bay  of  a  larger  lake,  various  aquatic  plants,  espe- 
cially the  species  of  pond  lilies,  take  root  upon  the  bot- 
tom, and  spread  their  expanded  leaves  on  the  surface  of 
the  water.  These  flexible-leaved  and  elastic-stemmed  plants 
can  endure  waves  which  attain  no  more  than  a  foot  or 
two  of  height,  and  by  the  friction  which  they  afford  make 
the  swash  on  the  shore  very  slight.  In  the  quiet  water, 
rushes  take  root,  and  still  further  protect  the  strand,  so 
that  the  very  delicate  vegetation  of  the  mosses,  such  as 
the  Sphagnum,  can  fix  itself  on  the  shore. 

As  soon  as  the  Sphagnum  mat  has  begun  its  growth, 
the  strength  given  by  its  interlaced  fibres  enables  it  to 
extend  off  from  the  shore  and  float  upon  the  water.  In 
this  way  it  may  rapidly  enlarge,  if  not  broken  up  by  the 


334:  OUTLINES  OF  THE  EARTH'S  HISTORY. 

waves,  so  that  its  front  advances  into  the  lake  at  the  rate 
of  several  inches  each  year.  While  growing  outwardly 
it  thickens,  so  that  the  bottom  of  the  mass  gradually 
works  down  toward  the  floor  of  the  basin.     At  the  same 


Fig.  17. — Diagram  showing  beginning  of  peat  bog :  a,  lake ;  b,  lilies 
and  rushes ;  c,  lake  bog ;  d,  climbing  bog. 

time  the  lower  part  of  the  sheet,  decaying,  contributes 
a  shower  of  soft  peat  mud  to  the  floor  of  the  lake.  In 
this  way,  growing  at  its  edge,  deepening,  and  contribut- 
ing to  an  upgrowth  from  the  bottom,  a  few  centuries 
may  serve  entirely  to  fill  a  deep  basin  with  peaty  accumu- 
lation. In  general,  however,  the  surface  of  the  bog  closes 
over  the  lake  before  the  accumulation  has  completely 
filled  the  shoreward  portions  of  the  area.  In  these  con- 
ditions we  have  what  is  familiarly  known  as  a  quaking 
bog,  which  can  be  swayed  up  and  down  by  a  person  who 
quickly  stoops  and  rises  while  •  standing  on  the  surface. 
In  this  state  the  tough  and  thick  sheet  of  growing  plants 
is  sufficient  to  uphold  a  considerable  weight,  but  so  elas- 
tic that  the  underlying  water  can  be  thrown  into  waves. 
Long  before  the  bog  has  completely  filled  the  lake  with 
the  peaty  accumulations  the  growth  of  trees  is  apt  to  take 
place  on  its  surface,  which  often  reduces  the  area  to  the 
appearance  of  a  very  level  wet  wood. 

Climbing  and  lake  bogs  in  the  United  States  occupy 
a  total  area  of  more  than  fifty  thousand  square  miles.  In 
all  North  America  the  total  area  is  probably  more  than 
twice  as  great.  Similar  deposits  are  exceedingly  common 
in  the  Eurasian  continent  and  in  southern  Patagonia.  It 
is  probable  that  the  total  amount  of  these  fields  in  differ- 


THE  SOIL. 


335 


ent  parts  of  the  world  exceeds  half  a  million  square  miles. 
These  two  groups  of  fresh-water  swamps  have  an  interest, 
for  the  reason  that  when  reduced  to  cultivation  by  drain- 
age and  by  subsequent  removal  of  the  excess  of  peaty 


Fig.  18. — Diagrtam  showing  development  of  swamp:  a,  remains  of 
lake  ;  b,  surface  growth ;  c,  peat. 


matter,  by  burning  or  by  natural  decay,  afford  very  rich 
soil.  The  fairest  fields  of  northern  Europe,  particularly 
in  Great  Britain  and  Ireland,  have  been  thus  won  to 
tillage.  In  the  first  centuries  of  our  era  a  large  part  of 
England — perhaps  as  much  as  one  tenth  of  the  ground 
now  tilled  in  that  country — was  occupied  by  these  lands, 
which  retained  water  in  such  measure  as  to  make  them 
unfit  for  tillage,  the  greater  portion  of  this  area  being 
in  the  condition  of  thin  climbing  bog.  For  many  cen- 
turies much  of  the  energy  of  the  people  was  devoted  to 
the  reclamation  of  these  valuable  lands.  This  task  of 
winning  the  swamp  lands  to  agriculture  has  been  more 
completely  accomplished  in  England  than  elsewhere,  but 
it  has  gone  far  on  the  continent  of  Europe,  particularly 
in  Germany.  In  the  United  States,  owing  to  the  fact 
that  lands  have  been  cheap,  little  of  this  work  of  swamp- 
draining  has  as  yet  been  accomplished.  It  is  likely  that 
the  next  great  field  of  improvement  to  be  cultivated  by 
the  enterprising  people  will  be  found  in  these  excessively 
humid  lands,  from  which  the  food-giving  resources  for 
the  support  of  many  million  people  can  be  won. 

The  group  of  marine  marshes  differs  in  many  impor- 
tant regards  from  those  which  are  formed  in  fresh  water. 


336  OUTLINES  OF  THE  EARTH'S  HISTORY. 

Where  the  tide  visits  any  coast  line,  and  in  sheltered  posi- 
tions along  that  shore,  a  number  of  plants,  mostly  be- 
longing to  the  group  of  grasses,  species  which  have  be- 
come accustomed  to  having  their  roots  bathed  by  salt 
water,  begin  the  formation  of  a  spongy  mat,  which  re- 
sembles that  composed  of  Sphagnum,  only  it  is  much 
more  solid.  This  mat  of  the  marine  marshes  soon  attains 
a  thickness  of  a  foot  or  more,  the  upper  or  growing  sur- 
face lying  in  a  position  where  it  is  covered  for  two  or 
three  hours  at  each  visit  of  the  tide.  Growing  rapidly 
outward  from  the  shore,  and  having  a  strength  which  en- 
ables it  to  resist  in  a  tolerably  effective  manner  waves  not 
more  than  two  or  three  feet  high,  this  accumulation  makes 
head  against  the  sea.  To  a  certain  extent  the  waves  under- 
mine the  front  of  the  sheet  and  break  up  masses  of  it, 
which  they  distribute  over  the  shallow  bottom  below  the 
level  at  which  these  plants  can  grow.  In  this  deeper  water, 
also,  other  marine  animals  and  plants  are  continually 
developing,  and  their  remains  are  added  to  the  accumula- 
tions which  are  ever  shallowing  the  water,  thus  permitting 
a  further  extension  of  the  level,  higher-lying  marsh.  This 
process  continues  until  the  growth  has  gone  as  far  as  the 
scouring  action  of  the  tidal  currents  will  permit.  In 
the  end  the  bay,  originally  of  wide-open  water,  is  only 
such  at  high  tide.  For  the  greater  part  of  the  time  it 
appears  as  broad  savannas,  whose  brilliant  green  gives 
them  the  aspect  of  rare  fertility. 

Owing  to  the  conditions  of  their  growth,  the  deposits 
formed  in  marine  marshes  contain  no  distinct  peat,  the 
nearest  approach  to  that  substance  being  the  tangle  of 
wirelike  roots  which  covers  the  upper  foot  or  so  of  the 
accumulation.  The  greater  part  of  the  mass  is  composed 
of  fine  silt,  brought  in  by  the  streams  of  land  water  which 
discharge  into  the  basin,  and  by  the  remains  of  animals 
which  dwelt  upon  the  bottom  or  between  the  stalks  of 
the  plants  that  occupy  the  surface  of  the  marshes.  These 
interspaces  afford  admirable  shelter  to  a  host  of  small 


THE  SOIL.  33Y 

marine  forms.  The  result  is,  that  the  tidal  marshes,  as 
well  as  the  lower-lying  mud  flats,  which  have  been  oc- 
cupied by  the  mat  of  vegetation,  afford  admirable  earth  for 
tillage.  Unfortunately,  however,  there  are  two  disadvan- 
tages connected  with  the  redemption  of  such  lands.  In 
the  first  place,  it  is  necessary  to  exclude  the  sea  from 
the  area,  which  can  only  be  accomplished  by  considerable 
engineering  work;  in  the  second  place,  the  exclusion  of 
the  tide  inevitably  results  in  the  silting  up  of  the  passage 
by  which  the  water  found  its  way  to  the  sea.  As  these 
openings  are  often  used  for  harbours,  the  effect  arising  from 
their  destruction  is  often  rather  serious.  Nevertheless,  in 
some  parts  of  the  world  very  extensive  and  most  fertile 
tracts  of  land  have  thus  been  won  from  the  sea;  a  large 
part  of  Holland  and  shore-land  districts  in  northern  Eu- 
rope are  made  up  of  fields  which  were  originally  covered 
by  the  tide.  Near  the  mouth  of  the  Ehine,  indeed,  the 
people  have  found  these  sea-bottom  soils  so  profitable 
that  they  have  gone  beyond  the  zone  of  the  marshes, 
and  have  drained  considerable  seas  which  of  old  were 
permanently  covered,  even  at  the  lowest  level  of  the 
waters. 

On  the  coast  of  North  America  marine  marshes  have 
an  extensive  development,  and  vary  much  in  character.  In 
the  Bay  of  Fundy,  where  the  tides  have  an  altitude  of 
fifty  feet  or  more,  the  energy  of  their  currents  is  such 
that  the  marsh  mat  rarely  forms.  Its  place,  however,  is 
taken  by  vast  and  ever-changing  mud  flats,  the  materials 
of  which  are  swept  to  and  fro  by  the  moving  waters. 
The  people  of  this  region  have  learned  an  art  of  a  peculiar 
nature,  by  which  they  win  broad  fields  of  excellent  land 
from  the  sea.  Selecting  an  area  of  the  fiats,  the  surface 
of  which  has  been  brought  to  within  a  few  feet  of  high 
tide,  they  inclose  it  with  a  stout  barrier  or  dike,  which 
has  openings  for  the  free  admission  of  the  tidal  waters. 
Entering  this  basin,  the  tide,  moving  with  considerable 
velocity,  bears  in  quantities  of  sediment.     In  the  basin. 


Fig.  19.— Map  of  Ipswich  marshes,  Massachusetts,  formed  behind  a 
barrier  beach. 


THE  SOIL.  339 

the  motion  being  arrested,  tliis  sediment  falls  to  the  bot- 
tom, and  serves  to  raise  its  level.  In  a  few  months  the 
sheet  of  sediment  is  brought  near  the  plane  of  the  tidal 
movement,  then  the  gates  are  closed  at  times  when  the 
tide  has  attained  half  of  its  height,  so  that  the  ground 
within  the  dike  is  not  visited  by  the  sea  water,  and  can 
be  cultivated. 

Along  the  coast  of  New  England  the  ordinary  marine 
marshes  attain  an  extensive  development  in  the  form  of 
broad-grassed  savannas.  With  this  aspect,  though  with 
a  considerable  change  in  the  plants  which  they  bear,  the 
fringe  of  savannas  continues  southward  along  the  coast 
to  northern  Florida.  In  the  region  about  the  mouth  of 
the  Savannah  River,  so  named  from  the  vast  extent  of 
the  tidal  marshes,  these  fields  attain  their  greatest  de- 
velopment. In  central  and  southern  Florida,  however, 
where  the  seacoast  is  admirably  suited  for  their  develop- 
ment, these  coastal  marshes  of  the  grassy  type  disappear, 
their  place  being  taken  by  the  peculiar  morasses  formed 
by  the  growth  of  the  mangrove  tree. 

'In  the  mangrove  marshes  the  tree  which  gives  the 
areas  their  name  covers  all  the  field  which  is  visited  by 
the  tide.  This  tree  grows  with  its  crown  supported  on 
stiltlike  roots,  at  a  level  above  high  tide.  From  its  hori- 
zontal branches  there  grow  off  roots,  which  reach  down- 
ward into  the  water,  and  thence  to  the  bottom.  The 
seeds  of  the  mangrove  are  admirably  devised  so  as  to 
enable  the  plant  to  obtain  a  foothold  on  the  mud  flats, 
even  where  they  are  covered  at  low  tide  with  a  depth  of 
two  or  three  feet  of  water.  They  are  several  inches  in 
length,  and  arranged  with  booklets  at  their  lower  ends; 
floating  near  the  bottom,  they  thus  catch  upon  it,  and  in 
a  few  weeks'  growth  push  the  shoot  to  the  level  of  the 
water,  thus  affording  a  foundation  for  a  new  plantation. 
fn  this  manner,  extending  the  old  forests  out  into  the 
shallow  water  of  the  bays,  and  forming  new  colonies 
wherever  the  water  is  not  too  deep,  these  plants  rapidly 


340 


OUTLINES  OF  THE  EARTH'S  HISTORY. 


occupy  all  the  region  which  elsewhere  would  appear  in 
the  form  of  savannas. 

The  tidal  marshes  of  North  America,  which  may  be 
in  time  converted  to  the  uses  of  man,  probably  occupy 
an  area  exceeding  twenty  thousand  square  miles.  If  the 
work  of  reclaiming  such  lands  from  the  sea  ever  attains 
the  advance  in  this  country  that  it  has  done  in  Holland, 
the  area  added  to  the  dry  land  by  engineering  devices 
may  amount  to  as  much  as  fifty  thousand  square  miles 
— a  territory  rather  greater  than  the   surface   of  Ken- 


— LowTide 


Sea  Floor 


Fig.  20. — Diagram  showing  mode  of  growth  of  mangroves. 


tucky,  and  with  a  food-yielding  power  at  least  five  times 
as  great  as  is  afforded  by  that  fertile  State.  In  fact,  these 
conquests  from  the  sea  are  hereafter  to  be  among  the 
great  works  which  will  attract  the  energies  of  mankind. 
In  the  arid  region  of  the  Cordilleras,  as  well  as  in 
many  other  countries,  the  soil,  though  destitute  of  those 
qualities  which  make  it  fit  for  the  uses  of  man,  because 
of  the  absence  of  water  in  sufficient  amount,  is,  as  regards 
its  structure  and  depth,  as  well  as  its  mineral  contents, 
admirably  suited  to  the  needs  of  agriculture.  The  de- 
velopment of  soils  in  desert  regions  is  in  almost  all 
cases  to  be  accounted  for  by  the  former  existence  in  the 
realms  they  occupy  of  a  much  greater  rainfall  than  now 
exists.     Thus  in  the  Rocky  Mountain  country,  when  the 


THE  SOIL.  341 

deep  soils  of  the  ample  valleys  were  formed,  the  lakes/  as 
we  have  before  noted,  were  no  longer  dead  seas,  as  is  at 
present  so  generally  the  ease,  but  poured  forth  great 
streams  to  the  sea.  Here,  as  elsewhere,  we  find  evidence 
that  certain  portions  of  the  earth  which  recently  had  an 
abundant  rainfall  have  now  become  starved  for  the  lack 
of  that  supply.  All  the  soils  of  arid  regions  where  the 
trial  has  been  made  have  proved  very  fertile  when  sub- 
jected to  irrigation,  which  can  often  be  accomplished  by 
storing  the  waters  of  the  brief  rainy  season  or  by  divert- 
ing those  of  rivers  which  enter  the  deserts  from  well- 
watered  mountain  fields.  In  fact,  the  soil  of  these  arid 
realms  yields  peculiarly  ample  returns  to  the  husbandman, 
because  of  certain  conditions  due  to  the  exceeding  dry- 
ness of  the  air.  This  leads  to  an  absence  of  cloudy 
weather,  so  that  from  the  time  the  seed  is  planted  the 
growth  is  stimulated  by  uninterrupted  and  intense  sun- 
shine. The  same  dryness  of  the  air  leads,  as  we  have 
seen,  to  a  rapid  evaporation  from  the  surface,  by  which, 
in  a  manner  before  noted,  the  dissolved  mineral  matter 
is  brought  near  the  top  of  the  soil,  where  it  can  best  serve 
the  greater  part  of  our  crop  plants.  On  these  accounts 
an  acre  of  irrigated  soil  can  be  made  to  yield  a  far  greater 
return  than  can  be  obtained  from  land  of  like  chemical 
composition  in  humid  regions. 

In  many  parts  of  the  world,  particularly  in  the  north- 
ern and  western  portions  of  the  Mississippi  Valley,  there 
are  widespread  areas,  which,  though  moderately  well 
watered,  were  in  their  virgin  state  almost  without  forests. 
In  the  prairie  region  the  early  settlers  found  the  coun- 
try unwooded,  except  along  the  margins  of  the  streams. 
On  the  borders  of  the  true  prairies,  however,  they  found 
considerable  areas  of  a  prevailingly  forested  land,  with 
here  and  there  a  tract  of  prairie.  There  were  several  of 
these  open  fields  south  of  the  Ohio,  though  the  country 
there  is  in  general  forested;  one  of  these  prairie  areas,  in 
the  Green  Eiver  district  of  Kentucky,  was  several  thou- 


342  OUTLINES  OF  THE  EARTH'S  HISTORY. 

sand  square  miles  in  extent.  At  first  it  was  supposed  that 
the  absence  of  trees  in  the  open  country  of  the  Mississippi 
Valley  was  due  to  some  peculiarity  of  the  soil,  but  experi- 
ence show^s  that  plantations  luxuriantly  develop,  and  that 
the  timber  will  spread  rapidly  in  the  natural  way.  In 
fact,  if  the  seeds  of  the  trees  which  have  been  planted 
since  the  settlement  of  the  country  were  allowed  to  de- 
velop as  they  seek  to  do,  it  would  only  be  a  few  centuries 
before  the  region  would  be  forest-clad  as  far  west  as  the 
rainfall  would  permit  the  plants  to  develop.  Probably  the 
woods  would  attain  to  near  the  hundredth  meridian. 

In  the  opinion  of  the  writer,  the  treeless  character  of 
the  Western  plains  is  mainly  to  be  accounted  for  by  the 
habit  which  our  Indians  had  of  burning  the  herbage  of 
a  lowly  sort  each  year,  so  that  the  large  game  might  ob- 
tain better  pasturage.  It  is  a  well-known  fact  to  all  those 
who  have  had  to  deal  with  cattle  on  fields  which  are  in 
the  natural  state  that  fire  betters  the  pasturage.  Begin- 
ning this  method  of  burning  in  the  arid  regions  to  the 
west  of  the  original  forests,  the  natural  action  of  the  fire 
has  been  gradually  to  destroy  these  woods.  Although 
the  older  and  larger  trees,  on  account  of  their  thick  bark 
and  the  height  of  their  foliage  above  the  ground,  escaped 
destruction,  all  the  smaller  and  younger  members  of  the 
species  were  constantly  swept  away.  Thus  when  the  old 
trees  died  they  left  no  succession,  and  the  country  assumed 
its  prairie  character.  That  the  prairies  were  formed  in 
this  manner  seems  to  be  proved  by  the  testimony  which 
we  have  concerning  the  open  area  before  mentioned  as 
having  existed  in  western  Kentucky.  It  is  said  that 
around  the  timberless  fields  there  was  a  wide  fringe  of 
old  fire-scarred  trees,  with  no  undergrowth  beneath  their 
branches,  and  that  as  they  died  no  kind  of  large  vegeta- 
tion took  their  place.  When  the  Indians  who  set  these 
fires  were  driven  away,  as  was  the  case  in  the  last  decade 
of  the  last  century,  the  country  at  once  began  to  resume 
its  timbered  condition.    From  the  margin  and  from  every 


THE  SOIL.  343 

interior  point  where  the  trees  survived,  their  seeds  spread 
so  that  before  the  open  land  was  all  subjugated  to  the 
plough  it  was  necessary  in  many  places  to  clear  away  a 
thick  growth  of  the  young  forest-building  trees. 

The  soils  which  develop  on  the  lavas  and  ashes  about 
an  active  volcano  afford  interesting  subjects  for  study, 
for  the  ,reason  that  they  show  how  far  the  development 
of  the  layer  which  supports  vegetation  may  depend  upon 
the  character  of  the  rocks  from  which  it  is  derived.  Where 
the  materials  ejected  from  a  volcano  lie  in  a  rainy  dis- 
trict, the  process  of  decay  which  converts  the  rock  into 
soil  is  commonly  very  rapid,  a  few  years  of  exposure  to 
the  weather  being  sufficient  to  bring  about  the  formation 
of  a  fertile  soil.  This  is  due  to  the  fact  that  most  lavas, 
as  well  as  the  so-called  volcanic  ashes,  which  are  of  the 
same  material  as  the  lavas,  only  blown  to  pieces,  are  com- 
posed of  varied  minerals,  the  most  of  which  are  readily 
attacked  by  the  agents  of  decay.  Now  and  then,  however, 
we  find  the  materials  ejected  from  a  particular  volcano,  or 
even  the  lavas  and  ashes  of  a  single  eruption,  in  such  a 
chemical  state  that  soils  form  upon  them  with  exceeding 
slowness. 

The  foregoing  incomplete  considerations  make  it  plairu.^ 
that  the  soil-covering  of  the  earth  is  the  result  of  very 
delicate  adjustments,  which  determine  the  rate  at  which 
the  broken-down  rocks  find  their  path  from  their  origi- 
nal bed  places  to  the  sea.  The  admirable  way  in  which 
this  movement  is  controlled  is  indicated  by  the  fact  that 
almost  everywhere  we  find  a  soil-covering  deep  enough 
for  the  use  of  a  varied  vegetation,  but  rarely  averaging 
more  than  a  dozen  feet  in  depth.  Only  here  and  there  are 
the  rocks  bare  or  the  earth  swathed  in  a  profound  mass 
of  detritus.  This  indicates  how  steadfast  and  measured  is 
the  march  of  the  rock  waste  from  the  hills  to  the  sea. 
Unhappily,  man,  when  by  his  needs  he  is  forced  to  till 
the  soil,  is  compelled  to  break  up  this  ancient  and  perfect 
23 


344  OUTLINES  OF  THE  EARTH'S  HISTORY. 

order.  He  has  to  strip  the  living  mantle  from  the  earth, 
replacing  it  with  growth  of  those  species  which  serve  his 
needs.  Those  plants  which  are  most  serviceable — which 
are,  indeed,  indispensable  in  the  higher  civilization,  the 
grains — require  for  their  cultivation  that  the  earth  be 
stripped  bare  and  deeply  stirred  during  the  rainy  season, 
and  thus  subjected  to  the  most  destructive  effect  of  the 
rainfall.  The  result  is,  that  in  almost  all  grain  fields  the 
rate  of  soil  destruction  vastly  surpasses  that  at  which  the 
accumulation  is  being  made.  We  may  say,  indeed,  that, 
except  in  alluvial  plains,  where  the  soil  grows  by  flood- 
made  additions  to  its  upper  surface,  no  field  tilled  in  grain 
can  without  exceeding  care  remain  usable  for  a  century. 
Even  though  the  agriculturist  returns  to  the  earth  all  the 
chemical  substances  which  he  takes  away  in  his  crops,  the 
loss  of  the  soil  by  the  washing  away  of  its  substance  to 
the  stream  will  inevitably  reduce  the  region  to  sterility. 

It  is  not  fanciful  to  say  that  the  greatest  misfortune 
which  in  a  large  way  man  has  had  to  meet  in  his  agri- 
culture arises  from  this  peculiar  stress  which  grain  crops 
put  upon  the  soil.  If  these  grains  grew  upon  perennial 
plants,  in  the  manner  of  our  larger  fruits,  the  problem  of 
man's  relation  to  the  soil  would  be  much  simpler  than  it 
is  at  present.  He  might  then  manage  to  till  the  earth 
without  bringing  upon  it  the  inevitable  destruction  which 
he  now  inflicts.  As  it  is,  he  should  recognise  that  his  needs 
imperil  this  ancient  and  precious  element  in  the  earth's 
structure,  and  he  should  endeavour  in  every  possible  way 
to  minimize  the  damage  which  he  brings  about.  This 
result  he  may  accomplish  in  certain  simple  ways. 

First,  as  regards  the  fertility  of  the  soil,  as  distin- 
guished from  the  thickness  of  the  coating,  it  may  be  said 
that  modern  discoveries  enable  us  to  see  the  ways  whereby 
we  may  for  an  indefinite  period  avoid  the  debasement  of 
our  great  heritage,  the  food-giving  earth.  We  now  know 
in  various  parts  of  the  world  extensive  and  practically 
inexhaustible  deposits,  whence  may  be  obtained  the  phos- 


THE  SOIL.  345 

phates,  potash,  soda,  etc.,  which  we  take  from  the  soil  in 
our  crops.  We  also  have  learned  ways  in  which  the  ma- 
terials contained  in  our  sewage  may  be  kept  from  the  sea 
and  restored  to  the  fields.  In  fact,  the  recent  developments 
of  agriculture  have  made  it  not  only  easy,  but  in  most 
cases  profitable,  to  avoid  this  waste  of  materials  which 
has  reduced  so  many  regions  to  poverty.  We  may  fairly 
look  forward  to  the  time,  not  long  distant,  when  the  old 
progressive  degradation  in  the  fertility  of  the  soil  coating 
will  no  longer  occur.  It  is  otherwise  with  the  mass  of  the 
soil,  that  body  of  commingled  decayed  rock  and  vegetable 
matter  which  must  possess  a  certain  thickness  in  order 
to  serve  its  needs.  As  yet  no  considerable  arrest  has  been 
made  in  the  processes  which  lead  to  the  destruction  of 
this  earthy  mass.  In  all  countries  where  tillage  is  gen- 
eral the  rivers  are  flowing  charged  with  all  they  can  bear 
away  of  soil  material.  Thus  in  the  valley  of  the  Po,  a 
region  where,  if  the  soil  were  forest-clad,  the  down-wearing 
of  the  surface  would  probably  be  at  no  greater  rate  than 
one  foot  m  five  thousand  years,  the  river  bears  away  the 
soil  detritus  so  rapidly  that  at  the  present  time  the  down- 
going  is  at  the  rate  of  one  foot  in  eight  hundred  years, 
and  each  decade  sees  the  soil  disappear  from  hillsides 
which  were  once  fertile,  but  are  now  reduced  to  bare 
rocks.  All  about  the  Mediterranean  the  traveller  notes 
extensive  regions  which  were  once  covered  with  luxuriant 
forests,  and  were  afterward  the  seats  of  prosperous  agri- 
culture, where  the  soil  has  utterly  disappeared,  leaving 
only  the  bare  rocks,  which  could  not  recover  its  natural 
covering  in  thousands  of  years  of  the  enforced  fallow. 

Within  the  limits  of  the  United  States  the  degrada- 
tion of  the  soil,  owing  to  the  peculiar  conditions  of  the 
country,  is  in  many  districts  going  forward  with  startling 
rapidity.  It  has  been  the  habit  of  our  people — a  habit 
favoured  by  the  wide  extent  of  fertile  and  easily  acquired 
frontier  ground — recklessly  to  till  their  farms  until  the 
fields  were  exhausted,  and  then  to  abandon  them  for  new 


346  OUTLINES  OF  THE  EARTH'S  HISTORY. 

ground. '  By  shallow  ploughing  on  steep  hillsides,  by  neg- 
lect in  the  beginning  of  those  gulches  which  form  in  such 
places,  it  is  easy  in  the  hill  country  of  the  eastern  United 
States  to  have  the  soil  washed  away  within  twenty  years 
after  the  protecting  forests  have  been  destroyed.  The 
writer  has  estimated  that  in  the  States  south  of  the  Ohio 
and  James  Kivers  more  than  eight  thousand  square  miles 
of  originally  fertile  ground  have  by  neglect  been  brought 
into  a  condition  where  it  will  no  longer  bear  crops  of  any 
kind,  and  over  fifteen  hundred  miles  of  the  area  have  been 
so  worn  down  to  the  subsoil  or  the  bed  rock  that  it  may 
never  be  profitable  to  win  it  again  to  agricultural  uses. 

Hitherto,  in  our  American  agriculture,  our  people  have 
been  to  a  great  extent  pioneers;  they  have  been  compelled 
to  win  what  they  could  in  the  cheapest  possible  way  and 
with  the  rudest  implements,  and  without  much  regard 
to  the  future  of  those  who  were  in  subsequent  genera- 
tions to  occupy  the  fields  which  they  were  conquering 
from  the  wilderness  and  the  savages.  The  danger  is  now 
that  this  reckless  tillage,  in  a  way  justified  of  old,  may 
be  continued  and  become  habitual  with  our  people.  It 
is,  indeed,  already  a  fixed  habit  in  many  parts  of  the 
country,  particularly  in  the  South,  where  a  small  farmer 
expects  to  wear  out  two  or  three  plantations  in  the  course 
of  his  natural  life.  Many  of  them  manage  to  ruin  from 
one  to  two  hundred  acres  of  land  in  the  course  of  half 
a  century  of  uninterrupted  labour.  This  system  deserves 
the  reprobation  of  all  good  citizens;  it  would  be  well, 
indeed,  if  it  were  possible  to  do  so,  to  stamp  it  out  by  the 
law.  The  same  principle  which  makes  it  illegal  for  a 
man  to  burn  his  own  dwelling  house  may  fairly  be  applied 
in  restraining  him  from  destroying  the  land  which  he 
tills. 

There  are  a  few  simple  principles  which,  if  properly 
applied,  may  serve  to  correct  this  misuse  of  our  American 
soil.  The  careful  tiller  should  note  that  all  soils  what- 
ever which  lie  on  declivities  having  a  slope  of  more  than 


THE  SOIL.  347 

one  foot  in  thirty  inevitably  and  rapidly  waste  when  sub- 
ject to  plough  tillage.  This  instrument  tends  to  smear 
and  consolidate  the  layer  of  earth  over  which  its  heel 
runs,  so  that  at  a  depth  of  a  few  inches  below  the  surface 
a  layer  tolerably  impervious  to  water  is  formed.  The 
result  is  that  the  porous  portion  of  the  deposit  becomes 
excessively  charged  with  water  in  times  of  heavy  rain, 
and  moves  down  the  hillside  in  a  rapid  manner.  All  such 
steep  slopes  should  be  left  in  their  wooded  state,  or,  if 
brought  into  use,  should  be  retained  as  pasture  lands. 

Where,  as  is  often  the  case  with  the  farms  in  hilly 
countries,  all  the  fields  are  steeply  inclined,  it  is  an  excel- 
lent precaution  to  leave  the  upper  part  of  the  slope  with 
a  forest  covering.  In  this  condition  not  only  is  the  ex- 
cessive flow  of  surface  water  diminished,  but  the  moisture 
which  creeps  down  the  slope  from  the  wooded  area  tends 
to  keep  the  lower-lying  fields  in  a  better  state  for  tillage, 
and  promotes  the  decay  of  the  underlying  rocks,  and  thus 
adds  to  the  body  and  richness  of  the  earth. 

On  those  soils  which  must  be  tilled,  even  where  they 
tend  to  wash  away,  the  aim  should  be  to  keep  the  detritus 
open  to  such  a  depth  that  it  may  take  in  as  much  as  pos- 
sible of  the  rainfall,  yielding  the  water  to  the  streams 
through  the  springs.  This  end  can  generally  be  accom- 
plished by  deep  ploughing;  it  can,  in  almost  all  cases,  be 
attained  by  under-drainage.  The  effect  of  allowing  the 
water  to  penetrate  is  not  only  to  diminish  the  superficial 
wearing,  but  to  maintain  the  process  of  subsoil  and  bed- 
rock decay  by  which  the  detrital  covering  is  naturally 
renewed.  Where,  as  in  many  parts  of  the  country,  the 
washing  away  of  the  soil  can  not  otherwise  be  arrested, 
the  progress  of  the  destruction  can  be  delayed  by  form- 
ing with  the  skilful  use  of  the  plough  ditches  of  slight 
declivity  leading  along  the  hillsides  to  the  natural  water- 
ways. One  of  the  most  satisfactory  marks  of  the  im- 
provement which  is  now  taking  place  in  the  agriculture 
of  the  cotton-yielding  States  of  this  country  is  to  be  found 


348  OUTLINES  OF  THE  EARTH'S  HISTORY. 

in  the  rapid  increase  in  the  use  of  the  ditch  system  here 
mentioned.  This  system,  combined  with  ploughing  in  the 
manner  where  the  earth  is  with  each  overturning  thrown 
uphill,  will  greatly  reduce  the  destructive  effect  of  rain- 
fall on  steep-lying  fields.  But  the  only  effective  protec- 
tion, however,  is  accomplished  by  carefully  terracing  the 
slopes,  so  that  the  tilled  ground  lies  in  level  benches.  This 
system  is  extensively  followed  in  the  thickly  settled  por- 
tions of  Europe,  but  it  may  be  a  century  before  it  will 
be  much  used  in  this  country. 

The  duty  of  the  soil-tiller  by  the  earth  with  which 
he  deals  may  be  briefly  summed  up:  He  should  look  upon 
himself  as  an  agent  necessarily  interfering  with  the  opera- 
tions which  naturally  form  and  preserve  the  soil.  He 
should  see  that  his  work  brings  two  risks;  he  may  impov- 
erish the  accumulation  of  detrital  material  by  taking  out 
the  plant  food  more  rapidly  than  it  is  prepared  for  use.  This 
injurious  result  may  be  at  any  time  reparable  by  a  proper 
use  of  manures.  Not  so,  however,  with  the  other  form 
of  destruction,  which  results  in  the  actual  removal  of 
the  soil  materials.  Where  neglect  has  brought  about  this 
disaster,  it  can  only  be  repaired  by  leaving  the  area  to 
recover  beneath  the  slowly  formed  forest  coating.  This 
process  in  almost  all  cases  requires  many  thousands  of 
years  for  its  accomplishment.  The  man  who  has  wrought 
such  destruction  has  harmed  the  inheritance  of  life. 


CHAPTER  IX. 

THE    ROCKS   AND   TfiEIR   ORDER. 

In  the  preceding  chapters  of  this  book  the  attention 
of  the  student  has  been  directed  mainly  to  the  opera- 
tions of  those  natural  forces  which  act  upon  the  surface 
of  the  earth.  Incidentally  the  consequences  arising  from 
the  applications  of  energy  to  the  outer  part  of  the  planet 
have  been  attended  to,  but  the  main  aim  has  been  to  set 
forth  the  work  which  solar  energy,  operating  in  the  form 
of  heat,  accomplishes  upon  the  lands.  We  have  now  to 
consider  one  of  the  great  results  of  these  actions,  which 
is  exhibited  in  the  successive  strata  that  make  up  the 
earth's  crust. 

The  most  noteworthy  effect  arising  from  the  action 
of  the  solar  forces  on  the  earth  and  their  co-operation 
with  those  which  originate  in  our  sphere  is  found  in  the 
destruction  of  beds  or  other  deposits  of  rock,  and  the 
removal  of  the  materials  to  the  floors  of  water  basins, 
where  they  are  again  aggregated  in  strata,  and  gradually 
brought  once  more  into  a  stable  condition  within  the 
earth.  This  work  is  accomplished  by  water  in  its  various 
states,  the  action  being  directly  affected  by  gravitation. 
In  the  form  of  steam,  water  which  has  been  built  into 
rocks  and  volcanically  expelled  by  tensions,  due  to  the 
heat  which  it  has  acquired  at  great  depths  below  the  sur- 
face, blows  forth  great  quantities  of  lava,  which  is  con- 
tributed to  the  formation  of  strata,  either  directly  in  the 
solid  form  or  indirectly,  after  having  been  dissolved  in 

349 


350  OUTLINES   OF  THE  EARTH'S  HISTORY. 

the  sea.  Acting  as  waves^  water  impelled  by  solar  energy 
transmitted  to  it  by  the  winds  beats  against  the  shores, 
wearing  away  great  quantities  of  rock,  which  is  dragged 
off  to  the  neighbouring  sea  bottoms,  there  to  resume  the 
bedded  form.  Moving  ice  in  glaciers,  water  again  apply- 
ing solar  energy  given  to  it  by  its  elevation  above  the 
sea,  most  effectively  grinds  away  the  elevated  parts  of 
the  crust,  the  debris  being  delivered  to  the  ocean.  In 
the  rain  the  same  work  is  done,  and  even  in  the  wind  the 
power  of  the  sun  serves  to  abrade  the  high-lying  rocks, 
making  new  strata  of  their  fragments. 

As  gravity  enters  as  an  element  in  all  the  movements 
of  divided  rock,  the  tendency  of  the  waste  worn  from  the 
land  is  to;*'gather  on  to  the  bottoms  of  basins  which  con- 
tain water.  Karely,  and  only  in  a  small  way,  this  process 
results  in  the  accumulation  of  lake  deposits;  the  greater 
part  of  the  work  is  done  upon  the  sea  floor.  When  the 
beds  are  formed  in  lake  basins,  they  may  be  accumulated 
in  either  of  two  very  diverse  conditions.  They  may  be 
formed  in  what  are  called  dead  seas,  in  which  case  the 
detrital  materials  are  commonly  small  in  amount,  for 
the  reason  that  the  inflowing  streams  are  inconsiderable; 
in  such  basins  there  is  normally  a  large  share  of  saline 
materials,  which  are  laid  down  by  the  evaporation  of  the 
water.  In  ordinary  lakes  the  deposits  which  are  formed 
are  mostly  due  to  the  sediment  that  the  rivers  import. 
These  materials  are  usually  fine-grained,  and  the  sand  or 
pebbles  which  they  contain  are  plentifully  mingled  with 
clay.  Hence  lake  deposits  are  usually  of  an  argillaceous 
nature.  As  organic  life,  such  as  secretes  limestone,  is 
rarely  developed  to  any  extent  in  lake  basins,  limy  beds 
are  very  rarely  formed  beneath  those  areas  of  water. 
Where  they  occur,  they  are  generally  due  to  the  fact  that 
rivers  charged  with  limy  matter  import  such  quantities 
of  the  substance  that  it  is  precipitated  on  the  bottom. 

As  lake  deposits  are  normally  formed  in  basins  above 
the  level  of  the  sea,  and  as  the  drainage  channels  of  the 


THE  ROCKS  AND  THEIR  ORDER.  351 

basins  are  always  cutting  down,  the  effect  is  to  leave 
such  strata  at  a  considerable  height  above  the  sea  level, 
where  the  erosive  agents  may  readily  attack  them.  In 
consequence  of  this  condition,  lacustrine  beds  are  rarely 
found  of  great  antiquity;  they  generally  disappear  soon 
after  they  are  formed.  Where  preserved,  their  endurance 
is  generally  to  be  attributed  to  the  fact  that  the  region 
they  occupy  has  been  lowered  beneath  the  sea  and  cov- 
ered by  marine  strata. 

The  great  laboratory  in  which  the  sedimentary  de- 
posits are  accumulated,  the  realm  in  which  at  least  ninety- 
nine  of  the  hundred  parts  of  these  materials  are  laid  down, 
is  the  oceanic  part  of  the  earth.  On  the  floors  of  the 
seas  and  oceans  we  have  not  only  the  region  where  the 
greater  part  of  the  sedimentation  is  effected,  but  that  in 
which  the  work  assumes  the  greatest  variety.  The  sea 
bottoms,  as  regards  the  deposits  formed  upon  them,  are 
naturally  divided  into  two  regions — the  one  in  which  the 
debris  from  the  land  forms  an  important  part  of  the  sedi- 
ment, and  the  other,  where  the  remoteness  of  the  shores 
deprives  the  sediment  of  land  w^aste,  or  at  least  of  enough 
of  that  material  in  any  such  share  as  can  affect  the  char- 
acter of  the  deposits. 

AVhat  we  may  term  the  littoral  or  shore  zone  of  the 
sea  occupies  a  belt  of  prevailingly  shallow  water,  varying 
in  width  from  a  few  score  to  a  few  hundred  miles.  Where 
the  bottom  descends  steeply  from  the  coast,  where  there 
are  no  strong  off-shore  setting  currents,  and  where  the 
region  is  not  near  the  mouth  of  a  large  river  which  bears 
a  great  tide  of  sediment  to  the  sea,  the  land  waste  may 
not  affect  the  bottom  for  more  than  a  mile  or  two  from 
the  shore.  Where  these  conditions  are  reversed,  the  debris 
from  the  air-covered  region  may  be  found  three  or  four 
hundred  miles  from  the  coast  line.  It  should  also  be 
noted  that  the  incessant  up-and-down  goings  of  the  land 
result  in  a  constant  change  in  the  position  of  the  coast 
line,  and  consequently  in  the  extension  of  the  land  sedi- 


352  OUTLINES  OF  THE  EARTH'S  HISTORY. 

ment,  in  the  course  of  a  few  geological  periods  over  a  far 
wider  field  of  sea  bottom  than  that  to  which  they  would 
attain  if  the  shores  remained  steadfast. 

It  is  characteristic  of  the  sediments  deposited  within 
the  influence  of  the  continental  detritus  that  they  vary  very 
much  in  their  action,  and  that  this  variation  takes  place 
not  only  horizontally  along  the  shores  in  the  same  stratum, 
but  vertically,  in  the  succession  of  the  beds.  It  also  may 
be  traced  down  the  slope  from  the  coast  line  to  deep 
water.  Thus  where  all  the  debris  comes  from  the  action 
of  the  waves,  the  deposits  formed  from  the  shore  out- 
wardly will  consist  of  coarse  materials,  such  as  pebbles 
near  the  coast,  of  sand  in  the  deeper  and  remoter  sec- 
tion, and  of  finer  silt  in  the  part  of  the  deposit  which  is 
farthest  out.  With  each  change  in  the  level  of  the  coast 
line  the  position  of  these  belts  will  necessarily  be  altered. 
Where  a  great  river  enters  the  sea,  the  changes  in  the 
volume  of  sediment  which  it  from  time  to  time  sends 
forth,  together  with  the  alternations  in  the  position  of  its 
point  of  discharge,  led  to  great  local  complexities  in  the 
strata.  Moreover,  the  turbid  water  sent  forth  by  the 
stream  may,  as  in  the  case  of  the  tide  from  the  Amazon, 
be  drifted  for  hundreds  of  miles  along  the  coast  line  or 
into  the  open  sea. 

The  most  important  variations  which  occur  in  the 
deposits  of  the  littoral  zone  are  brought  about  by  the 
formations  of  rocks  more  or  less  composed  of  limestone. 
Everywhere  the  sea  is,  as  compared  with  lake  waters,  re- 
markably rich  in  organic  life.  Next  the  shore,  partly 
because  the  water  is  there  shallow,  but  also  because  of 
its  relative  warmth  and  the  extent  to  which  it  is  in  mo- 
tion, organic  life,  both  that  of  animals  and  plants,  com- 
monly develops  in  a  very  luxuriant  way.  Only  where  the 
bottom  is  composed  of  drifting  sands,  which  do  not  afford 
a  foothold  for  those  species  which  need  to  rest  upon  the 
shore,  do  we  fail  to  find  that  surface  thickly  tenanted 
with  varied  forms.     These  are  arranged  according  to  the 


THE  ROCKS  AND  THEIR  ORDER.  353 

depth  of  the  bottom.  The  species  of  marine  plants  which 
are  attached  to  fixed  objects  are  limited  to  the  depth 
within  which  the  sunlight  effectively  penetrates  the  water; 
in  general,  it  may  be  said  that  they  do  not  extend  below 
a  depth  of  one  hundred  feet.  The  animal  forms  are  dis- 
tributed, according  to  their  kinds,  over  the  floor,  but  few 
species  having  the  capacity  to  endure  any  great  range 
in  the  pressure  of  the  sea  water.  Only  a  few  forms,  in- 
deed, extend  from  low  tide  to  the  depth  of  a  thousand 
feet. 

The  greatest  development  of  organic  life,  the  realm 
in  which  the  largest  number  of  species  occur,  and  where 
their  growth  is  most  rapid,  lies  within  about  a  hundred 
feet  of  the  low-tide  level.  Here  sunlight,  warmth,  and 
motion  in  the  water  combine  to  favour  organic  develop- 
ment. It  is  in  this  region  that  coral  reefs  and  other  great 
accumulations  of  limestone,  formed  from  the  skeletons 
of  polyps  and  mollusks,  most  abundantly  occur.  These 
deposits  of  a  limy  nature  depend  upon  a  very  delicate 
adjustment  of  the  conditions  which  favour  the  growth 
of  certain  creatures;  very  slight  geographic  changes,  by 
inducing  movements  of  sand  or  mud,  are  apt  to  interrupt 
their  formation,  bringing  about  a  great  and  immediate 
alteration  in  the  character  of  the  deposits.  Thus  it  is 
that  where  geologists  find  considerable  fields  of  rock, 
where  limestones  are  intercalated  with  sandstones  and  de- 
posits of  clay,  they  are  justified  in  assuming  that  the 
strata  were  laid  down  near  some  ancient  shore.  In  gen- 
eral, these  coast  deposits  become  more  and  more  limy  as 
we  go  toward  the  tropical  realms,  and  this  for  the  reason 
that  the  species  which  secrete  large  amounts  of  lime  are 
in  those  regions  most  abundant  and  attain  the  most  rapid 
growth.  The  stony  polyps,  the  most  vigorous  of  the  lime- 
stone makers,  grow  in  large  quantities  only  in  the  tropical 
realm,  or  near  to  it,  where  ocean  streams  of  great  warmth 
may  provide  the  creatures  with  the  conditions  of  tempera* 
ture  and  food  which  they  need. 


354  OUTLINES  OF  THE  EARTH'S  HISTORY. 

As  we  pass  from  the  shore  to  the  deeper  sea,  the  share 
of  land  detritus  rapidly  diminishes  until,  as  before  re- 
marked, at  the  distance  of  five  hundred  miles  from  the 
coast  line,  very  little  of  that  waste,  except  that  from  vol- 
canoes, attains  the  bottom  of  the  sea.  By  far  the  larger 
part  of  the  contributions  which  go  to  the  formation  of 
these  deep-sea  strata  come  from  organic  remains,  which 
are  continually  falling  upon  the  sea  floor.  In  part,  this 
waste  is  derived  from  creatures  which  dwell  upon  the 
bottom;  in  considerable  measure,  however,  it  is  from  the 
dead  bodies  of  those  forms  which  live  near  the  surface  of 
the  sea,  and  which  when  dying  sink  slowly  through  the 
intermediate  realm  to  the  bottom. 

Owing  to  the  absence  of  sunlight,  the  prevailingly 
cold  water  of  the  deeper  seas,  and  the  lack  of  vegeta- 
tion in  those  realms,  the  growth  of  organic  forms  on  the 
deep-sea  floor  is  relatively  slow.  Thus  it  happens  that 
each  shell  or  other  contribution  to  the  sediment  lies  for 
some  time  on  the  bottom  before  it  is  buried.  While  in 
this  condition  it  is  apt  to  be  devoured  by  some  of  the 
many  species  which  dwell  on  the  bottom  and  subsist  from 
the  remains  of  animals  and  plants  which  they  find  there. 
In  all  cases  the  fossilization  of  any  form  depends  upon 
the  accumulation  of  sediment  before  the  processes  of  de- 
struction have  overtaken  them,  and  among  these  processes 
we  must  give  the  first  place  to  the  creatures  which  sub- 
sist on  shells,  bones,  or  other  substances  of  like  nature 
which  find  their  way  to  the  ocean  floor.  In  the  absolute 
darkness,  the  still  water,  and  the  exceeding  cold  of  the 
deeper  seas,  animals  find  difficult  conditions  for  develop- 
ment. Moreover,  in  this  deep  realm  there  is  no  native 
vegetation,  and,  in  general,  but  little  material  of  this 
nature  descends  to  the  bottom  from  the  -surface  of  the  sea. 
The  result  is,  the  animals  have  to  subsist  on  the  remains 
of  other  animals  which  at  some  step  in  the  succession 
have  obtained  their  provender  from  the  plants  which 
belong  on  the  surface  or  in  the  shallow  waters  of  the  sea. 


THE  ROCKS  AND  THEIR  ORDER.  355 

This  limitation  of  the  food  supply  causes  the  depths  of 
the  sea  to  be  a  realm  of  continual  hunger,  a  region  where 
every  particle  of  organic  matter  is  apt  to  be  seized  upon 
by  some  needy  creature. 

In  consequence  of  the  fact  that  little  organic  matter 
on  the  deeper  sea  floors  escapes  being  devoured,  the  most 
of  the  material  of  this  nature  which  goes  into  strata  enters 
that  state  in  a  finely  divided  condition.  In  the  group  of 
worms  alone — forms  which  in  a  great  diversity  of  species 
inhabit  the  sea  floor — we  find  creatures  which  are  spe- 
cially adapted  to  digesting  the  debris  which  gathers  on 
the  sea  bottom.  Wandering  over  this  surface,  much  in 
the  manner  of  our  ordinary  earthworms,  these  creatures 
devour  the  mud,  voiding  the  matter  from  their  bodies  in 
a  yet  more  perfectly  divided  form.  Hence  it  comes  about 
that  the  limestone  beds,  so  commonly  formed  beneath 
the  open  seas,  are  generally  composed  of  materials  which 
show  but  few  and  very  imperfect  fossils.  Studying  any 
series  of  limestone  beds,  we  commonly  find  that  each 
layer,  in  greater  or  less  degree,  is  made  up  of  rather  mas- 
sive materials,  which  evidently  came  to  their  place  in 
the  form  of  a  limy  mud.  Very  often  this  lime  has  crys- 
tallized, and  thus  has  lost  all  trace  of  its  original  organic 
structure. 

One  of  the  conspicuous  features  which  may  be  ob- 
served in  any  succession  of  limestone  beds  is  the  partings 
or  divisions  into  layers  which  occur  with  varied  frequency. 
Sometimes  at  vertical  intervals  of  not  more  than  one  or 
two  inches,  again  with  spacings  of  a  score  of  feet,  we  find 
divisional  planes,  which  indicate  a  sudden  change  in  the 
process  of  rock  formation.  The  lime  disappears,  and  in 
place  of  it  we  have  a  thin  layer  of  very  fine  detritus, 
which  takes  on  the  form  of  a  clay.  Examining  these  part- 
ings with  care,  we  observe  that  on  the  upper  surface  on 
the  limestone  the  remains  of  the  animal  which  dwelt  on 
the  ancient  sea  floor  are  remarkably  well  preserved,  they 
having  evidently  escaped  the  effect  of  the  process  which 


356  OUTLINES  OF  THE  EARTH'S  HISTORY. 

reduced  their  ancestors,  whose  remains  constitute  the 
layer,  to  mud.  Furthermore,  we  note  that  the  shaly 
layer  is  not  only  lacking  in  lime,  but  commonly  contains 
no  trace  of  animals  such  as  might  have  dwelt  on  the  bot- 
tom. The  fossils  it  bears  are  usually  of  species  which  swam 
in  the  overlying  water  and  came  to  the  bottom  after  death. 
Following  up  through  the  layer  of  shale,  w^e  note  that  the 
ordinary  bottom  life  gradually  reappears,  and  shortly  be- 
comes so  plentiful  that  the  deposit  resumes  the  character 
which  it  had  before  the  interruption  began.  Often,  how- 
ever, we  note  that  the  assemblage  of  species  which  dwelt 
on  the  given  area  of  sea  floor  has  undergone  a  considerable 
change.  Forms  in  existence  in  the  lower  layer  may  be 
lacking  in  the  upper,  their  place  being  taken  by  new 
varieties. 

So  far  the  origin  of  these  divisional  planes  in  marine 
deposits  has  received  little  attention  from  geologists;  they 
have,  indeed,  assumed  that  each  of  these  alterations  indi- 
cates some  sudden  disturbance  of  the  life  of  the  sea  floors. 
They  have,  however,  generally  assumed  that  the  change 
was  due  to  alterations  in  the  depth  of  the  sea  or  in  the 
run  of  ocean  currents.  It  seems  to  the  writer,  however, 
that  while  these  divisions  may  in  certain  cases  be  due 
to  the  above-mentioned  and,  indeed,  to  a  great  variety 
of  causes,  they  are  in  general  best  to  be  explained  by  the 
action  of  earthquakes.  Water  being  an  exceedingly  elastic 
substance,  an  earthquake  passes  through  it  with  much 
greater  speed  than  it  traverses  the  rocks  which  support 
the  ocean  floor.  The  result  is  that,  when  the  fluid  and 
solid  oscillate  in  the  repeated  swingings  which  a  shock 
causes,  they  do  not  move  together,  but  rub  over  each 
other,  the  independent  movements  having  the  swing  of 
from  a  few  inches  to  a  foot  or  two  in  shocks  of  consider- 
able energy. 

When  the  sea  bottom  and  the  overlying  water,  vibrat- 
ing under  the  impulse  of  an  earthquake  shock,  move  past 
each  other,  the  inevitable  result  is  the  formation  of  muddy 


THE  ROCKS  AND  THEIR  ORDER.  357 

water;  the  very  fine  silt  of  the  bottom  is  shaken  up  into 
the  fluid,  which  afterward  descends  as  a  sheet  to  its  origi- 
nal position.  It  is  a  well-known  fact  that  such  muddying 
of  water,  in  which  species  .accustomed  to  other  conditions 
dwell,  inevitably  leads  to  their  death  by  covering  their 
breathing  organs  and  otherwise  disturbing  the  delicately 
balanced  conditions  which  enable  them  to  exist.  We 
find,  in  fact,  that  most  of  the  tenants  of  the  water,  par- 
ticularly the  forms  which  dwell  upon  the  bottom,  are 
provided  with  an  array  of  contrivances  which  enable  them 
to  clear  away  from  their  bodies  such  small  quantities  of 
silt  as  may  inconvenience  them.  Thus,  in  the  case  of  our 
common  clam,  the  breathing  organs  are  covered  with 
vibratory  cilia,  which,  acting  like  brooms,  sweep  off  any 
foreign  matter  which  may  come  upon  their  surfaces. 
Moreover,  the  creature  has  a  long,  double,  spoutlike  organ, 
which  it  can  elevate  some  distance  above  the  bottom, 
through  which  it  draws  and  discharges  the  water  from 
which  it  obtains  food  and  air.  Other  forms,  such  as  the 
crinoids,  or  sea  lilies,  elevate  the  breathing  parts  on  top 
of  tall  stems  of  marvellous  construction,  which  brings 
those  vital  organs  at  the  level,  it  may  be,  of  three  or  four 
feet  above  the  zone  of  mud.  In  consequence  of  the  peculiar 
method  of  growth,  the  crinoids  often  escape  the  damage 
done  by  the  disturbance  of  the  bottom,  and  thus  form 
limestone  beds  of  remarkable  thickness;  sometimes,  in- 
deed, we  find  these  layers  composed  mainly  of  crinoidal 
remains,  which  exhibit  only  slight  traces  of  partings  such 
as  we  have  described,  being  essentially  united  for  the 
depth  of  ten  or  twenty  feet.  Where  the  layers  have  been 
mainly  accumulated  by  shellfish,  their  average  thickness 
is  less  than  half  a  foot. 

When  we  examine  the  partitions  between  the  layers 
of  limestone,  we  commonly  find  that,  however  thin,  they 
generally  extend  for  an  indefinite  distance  in  every  direc- 
tion. The  writer  has  traced  some  of  these  for  miles;  never, 
indeed,  has  he  been  able  to  find  where  they  disappeared. 


358  OUTLIKES  OF  THE  EARTH'S  HISTORY. 

This  fact  makes  it  clear  that  the  destruction  which  took 
place  at  the  stage  where  these  partings  were  formed  was 
widespread;  so  far  as  it  was  due  to  earthquake  shocks,  we 
may  fairly  believe  that  in  many  cases  it  occurred  over 
areas  which  were  to  be  measured  by  tens  of  thousands  of 
square  miles.  Indeed,  from  what  we  know  of  earthquake 
shocks,  it  seems  likely  that  the  devastation  may  at  times 
have  affected  millions  of  square  miles. 

Another  class  of  accidents  connected  with  earthquakes 
may  also  suddenly  disturb  the  mud  on  the  sea  bottom. 
When,  as  elsewhere  noted,  a  shock  originates  beneath  the 
sea,  the  effect  is  suddenly  to  elevate  the  water  over  the 
seat  of  the  jarring  and  the  regions  thereabouts  to  the 
height  of  some  feet.  This  elevation  quickly  takes  the 
shape  of  a  ringlike  wave,  which  rolls  off  in  every  "direc- 
tion from  its  point  of  origin.  Where  the  sea  is  deep,  the 
effect  of  this  wave  on  the  bottom  may  be  but  slight;  but 
as  the  undulation  attains  shallower  water,  and  in  propor- 
tion to  the  shoaling,  the  front  of  the  surge  is  retarded 
in  its  advance  by  the  friction  of  the  bottom,  while  the 
rear  part,  being  in  deeper  water,  crowds  upon  the  ad- 
vancing line.  The  action  is  precisely  that  which  has 
been  described  as  occurring  in  wind-made  waves  as  they 
approach  the  beach;  but  in  this  last-named  group  of  un- 
dulations, because  of  the  great  width  of  the  swell,  the 
effect  of  the  shallowing  is  evident  in  much  deeper  water. 
It  is  likely  that  at  the  depth  of  a  thousand  feet  the  pass- 
ing of  one  of  these  vast  surges  born  of  earthquakes  may 
so  stir  the  mud  of  the  sea  floor  as  to  bring  about  a  wide- 
spread destruction  of  life,  and  thus  give  rise  to  many  of 
the  partitions  between  strata. 

If  we  examine  with  the  microscope  the  fine-grained 
silts  which  make  up  the  shaly  layers  between  limestones, 
we  find  the  materials  to  be  mostly  of  inorganic  origin. 
It  is  hard  to  trace  the  origin  of  the  mineral  matter  which 
it  contains;  some  of  the  fragments  are  likely  to  prove  of 
volcanic  origin;  others,  bits  of  dust  from  meteorites;  yet 


THE  ROCKS  AND  THEIR  ORDER.  359 

others,  dust  blown  from  the  land,  which  may,  as  we  know, 
be  conveyed  for  any  distance  across  the  seas.  Mingled 
with  this  sediment  of  an  inorganic  origin  we  almost  in- 
variably find  a  share  of  organic  waste,  derived  not  from 
creatures  which  dwelt  upon  the  bottom,  but  from  those 
which  inhabited  the  higher-lying  waters.  If,  now,  we  take 
a  portion  of  the  limestone  layer  which  lies  above  or  below 
the  shale  parting,  and  carefully  dissolve  out  with  acids 
the  limy  matter  which  it  contains,  we  obtain  a  residuum 
which  in  general  character,  except  so  far  as  the  particles 
may  have  been  affected  by  the  acid,  is  exactly  like  the 
material  which  forms  the  claylike  partition.  We  are  thus 
readily  led  to  the  conclusion  that  on  the  floors  of  the 
deeper  seas  there  is  constantly  descending,  in  the  form 
of  a  very  slow  shower,  a  mass  of  mineral  detritus.  Where 
organic  life  belonging  to  the  species  which  secrete  hard 
shells  or  skeletons  is  absent,  this  accumulation,  proceed- 
ing with  exceeding  slowness,  gradually  accumulates  layers, 
which  take  on  a  shaly  character.  Where  limestone-making 
animals  abound,  they  so  increase  the  rate  of  deposition 
that  the  proportion  of  th^  mineral  material  in  the  grow- 
ing strata  is  very  much  reduced;  it  may,  indeed,  become 
as  small  as  one  per  cent  of  the  mass.  In  this  case  we  may 
say  that  the  deposit  of  limestone  grew  a  hundred  times  as 
fast  as  the  intervening  beds  of  shale. 

The  foregoing  considerations  make  it  tolerably  clear 
that  the  sea  floor  is  in  receipt  of  two  diverse  classes  of 
sediment — those  of  a  mineral  and  those  of  an  organic 
origin.  The  mineral,  or  inorganic,  materials  predominate 
along  the  shores.  They  gradually  diminish  in  quantity 
toward  the  open  sea,  where  the  supply  is  mainly  depend- 
ent on  the  substances  thrown  forth  from  volcanoes,  on 
pumice  in  its  massive  or  its  comminuted  form — i.  e.,  vol- 
canic dust,  states  of  lava  in  which  the  material,  because 
of  the  vesicles  which  it  contains,  can  float  for  ages  before 
it  comes  to  rest  on  the  sea  bottom.  Variations  in  the  vol- 
canic waste  contributed  to  the  sea  floor  may  somewhat 
24 


360  OUTLINES  OF  THE  EARTH'S  HISTORY. 

affect  the  quantity  of  the  inorganic  sediments,  but,  as  a 
whole,  the  downfalling  of  these  fragments  is  probably  at 
a  singularly  uniform  rate.  It  is  otherwise  with  the  con- 
tributions of  sediment  arising  from  organic  forms.  This 
varies  in  a  surprising  measure.  On  the  coral  reefs,  such 
as  form  in  the  mid  oceans,  the  proportion  of  matter  which 
has  not  come  into  the  accumulatioix  through  the  bodies 
of  animals  and  plants  may  be  as  small  as  one  tenth  of  one 
per  cent,  or  less.  In  the  deeper  seas,  it  is  doubtful  whether 
the  rate  of  animal  growth  is  such  as  to  permit  the  forma- 
tion of  any  beds  which  have  less  than  one  half  of  their 
mass  made  up  of  materials  which  fell  througli  the  water. 

In  certain  areas  of  the  open  seas  the  upper  part  of 
the  water  is  dwelt  in  by  a  host  of  creatures,  mostly  forami- 
nifera,  which  extract  limestone  from  the  water,  and,  on 
dying,  send  their  shells  to  the  bottom.  Thus  in  the  North 
Atlantic,  even  where  the  sea  floor  is  of  great  depth  be- 
neath the  surface,  there  is  constantly  accumulating  a  mass 
of  limy  matter,  which  is  forming  very  massive  limestone 
strata,  somewhat  resembling  chalk  deposits,  such  as  abun- 
dantly occur  in  Great  Britain,  in  the  neighbouring  parts 
of  Europe,  in  Texas,  and  elsewhere.  Accumulations  such 
as  this,  where  the  supply  is  derived  from  the  surface  of 
the  water,  are  not  affected  by  the  accidents  which  divide 
beds  made  on  the  bottom  in  the  manner  before  described. 
They  may,  therefore,  have  the  singularly  continuous  char- 
acter which  we  note  in  the  English  chalk,  where,  for  the 
thickness  of  hundreds  of  feet,  we  may  have  no  evident 
partitions,  except  certain  divisions,  which  have  evidently 
originated  long  after  the  beds  were  formed. 

We  have  already  noted  the  fact  that,  while  the  floors 
of  the  deeper  seas  appear  to  lack  mountainous  elevations, 
those  arising  from  the  folding  of  strata,  they  are  plenti- 
fully scattered  over  with  volcanic  cones.  We  may  there- 
fore suppose  that,  in  general,  the  deposits  formed  on  the 
sea  floor  are  to  a  great  extent  affected  by  the  materials 
which  thes^  vents  cast  forth.     Lava  streams  and  showers 


THE  ROCKS  AND  THEIR  ORDER.  361 

represent  only  a  part  of  the  contributions  from  volcanoes, 
which  finally  find  their  way  to  the  bottom.  In  larger  part, 
the  materials  thrown  forth  are  probably  first  dissolved 
in  the  water  and  then  taken  up  by  the  organic  species; 
only  after  the  death  of  these  creatures  does  the  waste  go 
to  the  bottom.  As  hosts -of  these  creatures  have  no  solid 
skeleton  to  contribute  to  the  sea  floor,  such  mineral  matter 
as  they  may  obtain  is  after  their  death  at  once  restored  to 
the  sea. 

Not  only  does  the  contribution  of  organic  sediment 
diminish  in  quantity  with  the  depth  which  is  attained, 
but  the  deeper  parts  of  the  ocean  bed  appear  to  be  in  a 
condition  where  no  accumulations  of  this  nature  are  made, 
and  this  for  the  reason  that  the  water  dissolves  the  organic 
matter  more  rapidly  than  it  is  laid  down.  Thus  in  place 
of  limestone,  which  would  otherwise  form,  we  have  only 
a  claylike  residuum,  such  as  is  obtained  when  we  dissolve 
lime  rocks  in  acids.  This  process  of  solution,  by  which 
the  limy  matter  deposited  on  the  bottom  is  taken  back  into 
the  water,  goes  on  everywhere,  but  at  a  rate  which  in- 
creases with  the  depth.  This  increase  is  due  in  part  to 
the  augmentation  of  pressure,  and  in  part  to  the  larger 
share  of  carbonic  dioxide  which  the  water  at  great  depths 
holds.  The  result  is,  that  explorations  with  the  dredge 
seem  to  indicate  that  on  certain  parts  of  the  deeper  sea 
floors  the  rocks  are  undergoing  a  process  of  dissolution 
comparable  to  that  which  takes  place  in  limestone  caverns. 
So  considerable  is  the  solvent  work  that  a  large  part  of  the 
inorganic  waste  appears  to  be  taken  up  by  the  waters,  so 
as  to  leave  the  bottom  essentially  without  sedimentary 
accumulations.  The  sea,  in  a  word,  appears  to  be  eating 
into  rocks  which  it  laid  down  before  the  depression  at- 
tained its  present  great  depth. 

We  should  here  note  something  of  the  conditions  which 
determine  the  supply  of  food  which  the  marine  animals 
obtain.  First  of  all,  we  may  recur  to  the  point  that  the 
ocean  waters  appear  to  contain  something  of  all  the  earth 


362  OUTLINES  OF  THE  EARTH'S  HISTORY. 

materials  which  do  not  readily  decompose  when  they  are 
taken  into  the  state  of  solution.  These  mineral  substances, 
including  the  metals,  are  obtained  in  part  from  the  lands, 
through  the  action  of  the  rain  water  and  the  waves,  but 
perhaps  in  larger  share  from  the  volcanic  matter  which, 
in  the  form  of  floating  lava,  pumice,  or  dust,  is  plentifully 
delivered  to  the  sea.  Except  doubtfully,  and  at  most  in 
a  very  small  way,  this  chemical  store  of  the  sea  water  can 
not  be  directly  taken  into  the  structures  of  animals;  it 
can  only  be  immediately  appropriated  by  the  marine 
plants.  These  forms  can  only  develop  in  that  superficial 
realm  of  the  seas  which  is  penetrated  by  the  sunlight,  or 
say  within  the  depth  of  five  hundred  feet,  mostly  within 
one  hundred  feet  of  the  surface,  about  one  thirtieth  of  the 
average,  and  about  one  fiftieth  of  the  maximum  ocean 
depth.  On  this  marine  plant  life,  and  in  a  small  measure 
on  the  vegetable  matter  derived  from  the  land,  the  marine 
animals  primarily  depend  for  their  provender.  Through 
the  conditions  which  bring  about  the  formation  of  Sar- 
gassum  seas,  those  areas  of  the  ocean  where  seaweeds  grow 
afloat,  as  well  as  by  the  water-logging  and  weighting  down 
of  other  vegetable  matter,  some  part  of  the  plant  remains 
is  carried  to  the  sea  floor,  even  to  great  depths;  but  the 
main  dependence  of  the  deep-sea  forms  of  animals  is  upon 
other  animal  forms,  which  themselves  may  have  obtained 
their  store  from  yet  others.  In  fact,  in  any  deep-sea  form 
we  might  find  it  necessary  to  trace  back  the  food  by  thou- 
sands of  steps  before  we  found  the  creature  which  had 
access  to  the  vegetable  matter.  It  is  easy  to  see  how  such 
conditions  profoundly  limit  the  development  of  organic 
being  in  the  abysm  of  the  ocean. 

The  sedentary  animals,  or  those  which  are  fixed  to 
the  sea  bottom — a  group  which  includes  the  larger  part 
of  the  marine  species — ^have  to  depend  for  their  suste- 
nance on  the  movement  of  the  water  which  passes  their 
station.  If  the  seas  were  perfectly  still,  none  of  these 
creatures  except  the  most  minute  could  be  fed;  therefore 


THE  ROCKS  AND  THEIR  ORDER.  363 

the  currents  of  the  ocean  go  far  by  their  speed  to  deter- 
mine the  rate  at  which  life  may  flourish.  At  great  depths, 
as  we  have  seen,  these  movements  are  practically  limited 
to  that  which  is  caused  by  the  slow  movement  which  the 
tide  brings  about.  The  amount  of  this  motion  is  propor- 
tional to  the  depth  of  the  sea;  in  the  deeper  parts,  it  car- 
ries the  water  to  and  fro  twice  each  day  for  the  distance  of 
about  two  hundred  and  fifty  feet.  In  the  shallower  water 
this  motion  increases  in  proportion  to  the  shoaling,  and 
in  the  regions  near  the  shores  the  currents  of  the  sea 
which,  except  the  massive  drift  from  the  poles,  do  not 
usually  touch  the  bottom,  begin  to  have  their  influence. 
Where  the  water  is  less  than  a  hundred  feet  in  depth,  each 
wave  contributes  to  the  movement,  which  attains  its  maxi- 
mum near  the  shore,  where  every  surge  sweeps  the  water 
rapidly  to  and  fro.  It  is  in  this  surge  belt,  where  the 
waves  are  broken,  that  marine  animals  are  best  provided 
with  food,  and  it  is  here  that  their  growth  is  most  rapid. 
If  the  student  will  obtain  a  pint  of  water  from  the  surf, 
he  will  find  that  it  is  clouded  by  fragments  of  organic 
matter,  the  quantity  in  a  pound  of  the  fluid  often  amount- 
ing to  the  fiftieth  part  of  its  weight.  He  will  thus  per- 
ceive that  along  the  shore  line,  though  the  provision  of 
victuals  is  most  abundant,  the  store  is  made  from  the  ani- 
mals and  plants  which  are  ground  up  in  the  mill.  In  a 
word,  while  the  coast  is  a  place  of  rapid  growth,  it  is 
also  a  region  of  rapid  destruction;  only  in  the  case  of  the 
coral  animals,  which  associate  their  bodies  with  a  number 
of  myriads  in  large  and  elaborately  organized  communities, 
do  we  find  animals  which  can  make  such  head  against  the 
action  of  the  waves  that  they  can  build  great  deposits  in 
their  realm. 

It  should  be  noted  that  a  part  of  the  advantage  which 
is  afforded  to  organic  life  by  the  shore  belt  is  due  to  the 
fact  that  the  waters  are  there  subjected  to  a  constant 
process  of  aeration  by  the  whipping  into  foam  and  spray 
which  occurs  where  the  waves  overturn. 


364  OUTLINES  OF  THE  EARTH'S  HISTORY. 

It  will  be  interesting  to  the  student  to  note  the  great 
number  of  mechanical  contrivances  which  have  been  de- 
vised to  give  security  to  animals  and  plants  which  face 
these  difficult  conditions  arising  from  successive  violent 
blows  of  falling  water.  Among  these  may  be  briefly  noted 
those  of  the  limpets — mollusks  which  dwell  in  a  conical 
shell,  which  faces  the  water  with  a  domelike  outside,  and 
which  at  the  moment  of  the  stroke  is  drawn  down  upon 
the  rock  by  the  strong  muscle  which  fastens  the  creature 
to  its  foundation.  The  barnacles,  which  with  their  wedge- 
shaped  prows  cut  the  water  at  the  moment  of  the  stroke, 
but  open  in  the  pauses  between  the  waves,  so  that  the  crea- 
ture may  with  its  branching  arms  grasp  at  the  food  which 
floats  about  it;  the  nullipores,  forms  of  seaweed  which  are 
framed  of  limestone  and  cling  firmly  to  the  rock — afford 
yet  other  instances  of  protective  adaptations  contrived 
to  insure  the  safety  of  creatures  which  dwell  in  the  field 
of  abundant  food  supply. 

The  facts  above  presented  will  show  the  reader  that 
the  marine  sediments  are  formed  under  conditions  which 
permit  a  great  variety  in  the  nature  of  the  materials  of 
which  they  are  composed.  As  soon  as  the  deposits  are 
built  into  rocks  and  covered  by  later  accumulations,  their 
materials  enter  the  laboratory  of  the  under  earth,  where 
they  are  subjected  to  progressive  changes.  Even  before 
they  have  attained  a  great  depth,  through  the  laying  down 
of  later  deposits  upon  them,  changes  begin  which  serve 
to  alter  their  structure.  The  fragments  of  a  soluble  kind 
begin  to  be  dissolved,  and  are  redeposited,  so  that  the  mass 
commonly  becomes  much  more  solid,  passing  from  the 
state  of  detritus  to  that  of  more  or  less  solid  rock.  When 
yet  more  deeply  buried,  and  thereby  brought  into  a  realm 
of  greater  warmth,  or  perhaps  when  penetrated  by  dikes 
and  thereby  heated,  these  changes  go  yet  further.  More 
of  the  material  is  commonly  rearranged  by  solution  and 
redeposition,   so   that  limestone   may  be   converted   into 


THE  ROCKS  AND  THEIR  ORDER.  365 

crystalline  marble,  granular  sandstones  into  firm  masses, 
known  as  quartzites,  and  clays  into  the  harder  form  of 
slate.  Where  the  changes  go  to  the  extreme  j)oint,  rocks 
originally  distinctly  bedded  probably  may  be  so  taken  to 
pieces  and  made  over  that  all  traces  of  their  stratification 
may  be  destroyed,  all  fossils  obliterated,  and  the  stone 
transformed  into  mica  schist,  or  granite  or  other  crys- 
talline rock.  It  may  be  injected  into  the  overlying  strata 
in  the  form  of  dikes,  or  it  may  be  blown  forth  into  the 
air  through  volcanoes.  Involved  in  mountain-folding, 
after  being  more  or  less  changed  in  the  manner  described, 
the  beds  may  become  tangled  together  like  the  rumpled 
leaves  of  a  book,  or  even  with  the  complexity  of  snarled 
thread.  All  these  changes  of  condition  makes  it  difficult 
for  the  geologist  to  unravel  the  succession  of  strata  so 
that  he  may  know  the  true  order  of  the  rocks,  and  read 
from  them  the  story  of  the  successive  geological  periods. 
This  task,  though  incomplete,  has  by  the  labours  of  many 
thousand  men  been  so  far  advanced  that  we  are  now  able 
to  divide  the  record  into  chapters,  the  divisions  of  the 
geologic  ages,  and  to  give  some  account  of  the  succession 
of  events,  organic  and  geographic,  which  have  occurred 
since  life  began  to  write  its  records.. 

Eakthquakes. 

In  ordinary  experience  we  seem  to  behold  the  greater 
part  of  the  earth  which  meets  our  eyes  as  fixed  in  its  posi- 
tion. A  better  understanding  shows  us  that  nothing  in 
this  world  is  immovable.  In  the  realm  of  the  inorganic 
world  the  atoms  and  molecules  even  in  solid  bodies  have 
to  be  conceived  as  endowed  with  ceaseless  though  ordered 
motions.  Even  when  matter  is  built  into  the  solid  rock, 
it  is  doubtful  whether  any  grain  of  it  ever  comes  really 
to  rest.  Under  the  strains  which  arise  from  the  contrac- 
tion of  the  earth's  interior  and  the  chemical  changes  which 
the  rocks  undergo,  each  bit  is  subject  to  ever-changing 


366  OUTLINES  OF  THE  EARTH'S  HISTORY. 

thrusts,  wliicli  somewhat  affect  its  position.  If  we  in  any 
way  could  bring  a  grain  of  sand  from  any  stratum  under 
a  microscope,  so  that  we  could  perceive  its  changes  of 
place,  we  should  probably  find  that  it  was  endlessly  sway- 
ing this  way  and  that,  with  reference  to  an  ideally  fixed 
point,  such  as  the  centre  of  the  earth.  But  even  that 
centre,  whether  of  gravity  or  of  figure,  is  probably  never 
at  rest. 

Earth  movements  may  be  divided  into  two  groups — 
those  which  arise  from  the  bodily  shifting  of  matter,  which 
conveys  the  particles  this  way  or  that,  or,  as  we  say,  change 
their  place,  and  those  which  merely  produce  vibration,  in 
which  the  particles,  after  their  vibratory  movement,  re- 
turn to  their  original  place.  For  purposes  of  illustra- 
tion the  first,  or  translatory  motion,  may  be  compared  to 
that  which  takes  place  when  a  bell  is  carried  along  upon 
a  locomotive  or  a  ship;  and  the  second,  or  vibratory  move- 
ment, to  what  takes  place  when  the  bell  is  by  a  blow  made 
to  ring.  It  is  with  these  ringing  movements,  as  we  may 
term  them,  that  we  find  ourselves  concerned  when  we 
undertake  the  study  of  earthquakes. 

It  is  desirable  that  the  reader  should  preface  his  study 
of  earthquakes  by  noting  the  great  and,  at  the  same  time, 
variable  elasticity  of.  rocks.  In  the  extreme  form  this  elas- 
ticity is  very  well  shown  when  a  toy  marble,  which  is 
made  of  a  close-textured  rock,  such  as  that  from  which 
it  derives  its  name,  is  thrown  upon  a  pavement  composed 
of  like  dense  material.  Experiment  will  show  that  the 
little  sphere  can  often  be  made  to  bounce  to  the  height 
of  twenty  feet  without  breaking.  If,  then,  with  the  same 
energy  the  marble  is  thrown  upon  a  brick  floor,  the  re- 
bound will  be  very  much  diminished.  It  is  well  to  con- 
sider what  happens  to  produce  the  rebound.  When  the 
sphere  strikes  the  floor  it  changes  its  shape,  becoming 
shorter  in  the  axis  at  right  angles  to  the  point  which 
was  struck,  and  at  the  same  instant  expanded  along  the 
equator  of  that  axis.     The  flattening  remains  for  only  a 


THE  ROCKS  AND  THEIR  ORDER.  367 

small  fraction  of  a  second;  the  sphere  vibrates  so  that  it 
stretches  along  the  line  on  which  it  previously  shortened, 
and,  as  this  movement  takes  place  with  great  swiftness, 
it  may  be  said  to  propel  itself  away  from  the  floor.  At 
the  same  time  a  similar  movement  goes  on  in  the  rock 
of  the  floor,  and,  where  the  rate  of  vibration  is  the  same, 
the  two  kicks  are  coincident,  and  so  the  sphere  is  impelled 
violently  away  from  the  point  of  contact.  Where  the 
marble  comes  in  contact  with  brick,  in  part  because  of 
the  lesser  elasticity  of  that  material,  due  to  its  rather 
porous  structure,  and  partly  because  it  does  not  vibrate  at 
the  same  rate  as  the  marble,  the  expelling  blow  is  much 
less  strong. 

All  rocks  whatever,  even  those  which  appear  as  inco- 
herent sands,  are  more  or  less  set  into  vibratory  motion 
whenever  they  are  struck  by  a  blow.  In  the  crust  of  the 
earth  various  accidents  occur  which  may  produce  that  sud- 
den motion  which  we  term  a  blow.  When  we  have  exam- 
ined into  the  origin  of  these  impulses,  and  the  way  in  which 
they  are  transmitted  through  the  rocks,  we  obtain  a  basis 
for  understanding  earthquake  shocks.  The  commonest 
cause  of  the  jarrings  in  the  earth  is  found  in  the  formation 
of  fractures,  known  as  faults.  If  the  reader  has  ever  been 
upon  a  frozen  lake  at  a  time  when  the  weather  was  growing 
colder,  and  the  ice,  therefore,  was  shrinking,  he  may  have 
noted  the  rending  sound  and  the  slight  vibration  which 
comes  with  the  formation  of  a  crack  traversing  the  sheet 
of  ice.  At  such  a  time  he  feels  a  movement  which  is  an 
earthquake,  and  which  represents  the  simpler  form  of 
those  tremors  arising  from  the  sudden  rupture  of  fault 
planes.  If  he  has  a  mind  to  make  the  experiment,  he  may 
hang  a  bullet  by  a  thread  from  a  small  frame  which  rests 
upon  the  ice,  and  note  that  as  the  vibration  occurs  the 
little  pendulum  sways  to  and  fro,  thus  indicating  the 
oscillations  of  the  ice.  The  same  instrument  will  move 
in  an  identical  manner  when  affected  by  a  quaking  in 
the  rocks. 


368  OUTLINES  OF  THE  EARTH'S  HISTORY. 

Where  the  rocks  are  set  in  vibration  by  a  rent  which  is 
formed  in  them,  the  phenomena  are  more  complicated, 
and  often  on  a  vastly  larger  scale  than  in  the  simple  con- 
ditions afforded  by  a  sheet  of  ice.  The  rocks  on  either 
side  of  the  rupture  generally  slide  over  each  other,  and 
the  opposing  masses  are  rent  in  their  friction  upon  one 
another;  the  result  is,  not  only  the  first  jar  formed  by 
the  initial  fracture,  but  a  great  many  successive  move- 
ments from  the  other  breakages  which  occur.  Again,  in 
the  deeper  parts  of  the  crust,  the  fault  fissures  are  often 
at  the  moment  of  their  formation  filled  by  a  violent  in- 
rush of  liquid  rock-  This,  as  it  swiftly  moves  along,  tears 
away  masses  from  the  walls,  and  when  it  strikes  the  end 
of  the  opening  delivers  a  blow  which  may  be  of  great  vio- 
lence. The  nature  of  this  stroke  may  be  judged  by  the 
familiar  instance  where  the  relatively  slow-flowing  stream 
from  a  hydrant  pipe  is  suddenly  choked  by  closing  the 
stopcock.  Unless  the  plumber  provides  a  cushion  of  air 
to  diminish  the  energy  of  the  blow,  it  is  often  strong 
enough  to  shake  the  house.  Again,  when  steam  or  other 
gases  are  by  a  sudden  diminution  of  pressure  enabled  to 
expand,  they  may  deliver  a  blow  which  is  exactly  like  that 
caused  by  the  explosion  of  gunpowder,  which,  even  when 
it  rushes  against  the  soft  cushion  of  the  air,  may  cause 
a  jarring  that  may  be  felt  as  well  as  heard  to  a  great  dis- 
tance. Such  movements  very  frequently  occur  in  the  erup- 
tions of  volcanoes;  they  cause  a  quivering  of  the  earth, 
which  may  be  felt  for  a  great  distance  from  the  immediate 
seat  of  the  disturbance. 

When  by  any  of  the  sudden  movements  which  have 
been  above  described  a  jar  is  applied  to  the  rocks,  the 
wave  flies  through  the  more  or  less  elastic  mass  until  the 
energy  involved  in  it  is  exhausted.  This  may  not  be 
brought  about  until  the  motion  has  travelled  for  the  dis- 
tance of  hundreds  of  miles.  In  the  great  earthquake  of 
1755,  known  as  the  Lisbon  shock,  the  records  make  it 
seem  probable  that  the  movement  was  felt  over  one  eighth 


THE  ROCKS  AND  THEIR  ORDER.  369 

part  of  the  earth's  surface.  Such  great  disturbances  prob- 
ably bring  about  a  motion  of  the  rocks  near  the  point  of 
origin,  which  may  be  expressed  in  oscillations  having  an 
amplitude  of  one  to  two  feet;  but  in  the  greater  number 
of  earthquakes  the  maximum  swing  probably  does  not 
exceed  the  tenth  of  that  amount.  Very  sensible  shaking, 
even  such  as  may  produce  considerable  damage  to  build- 
ings, are  caused  by  shocks  in  which  the  earth  vibrates 
with  less  than  an  inch  of  swing. 

When  a  shock  originates,  the  wave  in  the  rocks  due 
to  the  compression  which  the  blow  inflicts  runs  at  a  speed 
varying  with  the  elasticity  of  the  substance,  but  at  the 
rate  of  about  fifteen  hundred  feet  a  second.  The  move- 
ments of  this  wave  are  at  right  angles  to  the  seat  of  the 
originating  disturbance,  so  that  the  shock  may  come  to 
the  surface  in  a  line  forming  any  angle  between  the  verti- 
cal and  the  nearly  horizontal.  Where,  as  in  a  volcanic 
eruption,  the  shock  originates  with  an  explosion,  these 
waves  go  off  in  circles.  Where,  however,  as  is  generally 
the  case,  the  shock  originates  in  a  fault  plane,  which  may 
have  a  length  and  depth  of  many  miles,  the  movement  has 
an  elliptical  form. 

If  the  earthquake  wave  ran  through  a  uniform  and 
highly  elastic  substance,  such  as  glass,  it  would  move 
everywhere  with  equal  speed,  and,  in  the  case  of  the 
greater  disturbances,  the  motion  might  be  felt  over  the 
whole  surface  of  the  earth.  But  as  the  motion  takes  place 
through  rocks  of  varying  elasticity,  the  rate  at  which  it 
journeys  is  very  irregular.  Moving  through  materials  of 
one  density,  and  with  a  rate  of  vibration  determined  by 
those  conditions,  the  impulse  is  with  difficulty  communi- 
cated to  strata  which  naturally  vibrate  at  another  speed. 
In  many  cases,  as  where  a  shock  passing  through  dense 
crystalline  strata  encounters  a  mass  of  soft  sandstone,  the 
wave,  in  place  of  going  on,  is  reflected  back  toward  its 
point  of  origin.  These  earthquake  echoes  sometimes  give 
rise  to  very  destructive  movements.    It  often  happens  that 


370  OUTLINES  OF  THE  EARTH'S  HISTORY. 

before  the  original  tremors  of  a  shock  have  passed  away 
from  a  point  on  the  surface  the  reflex  movements  rush 
in,  making  a  very  irregular  motion,  which  may  he  com- 
pared to  that  of  the  waves  in  a  cross-sea. 

The  foregoing  account  of  earthquake  action  will  serve 
to  prepare  the  reader  for  an  understanding  of  those  very 
curious  and  important  effects  which  these  accidents  pro- 
duce in  and  on  the  earth.  Below  the  surface  the  sensible 
action  of  earthquake  shocks  is  limited.  It  has  often  been 
observed  that  people  in  mines  hardly  note  a  swaying 
which  may  be  very  conspicuous  to  those  on  the  surface, 
the  reason  for  this  being  that  underground,  where  the 
rocks  are  firmly  bound  together,  all  those  swingings  which 
are  due  to  the  unsupported  position  of  such  objects  as 
buildings,  columnar  rocks,  trees,  and  the  waters  of  the 
earth,  are  absent.  The  effect  of  the  movements  which 
earthquakes  impress  on  the  under  earth  is  mainly  due 
to  the  fact  that  in  almost  every  part  of  the  crust  tensions 
or  strains  of  other  kinds  are  continually  forming.  These 
may  for  ages  prove  without  effect  until  the  earth  is  jarred, 
when  motions  will  suddenly  take  place  which  in  a  moment 
may  alter  the  conditions  of  the  rocks  throughout  a  wide 
field.  In  a  word,  a  great  earthquake  caused  by  the  forma- 
tion of  an  extensive  fault  is  likely  to  produce  any  number 
of  slight  dislocations,  each  of  which  is  in  turn  shock- 
making,  sending  its  little  wave  to  complicate  the  great 
oscillation.  Nor  does  the  perturbing  effect  of  these  jar- 
ring movements  cease  with  the  fractures  which  they  set 
up  and  the  new  strains  which  are  in  turn  developed  by 
the  motions  which  they  induce.  The  alterations  of  the 
rocks  which  are  involved  in  chemical  changes  are  favoured 
by  such  motions.  It  is  a  familiar  experience  that  a  vessel 
of  water,  if  kept  in  the  state  of  repose,  may  have  its  tem- 
perature lowered  three  or  four  degrees  below  the  freezing 
point  without  becoming  frozen.  If  the  side  of  th^  vessel  is 
then  tapped  with  the  finger,  so  as  to  send  a  slight  quake 
through  the  mass,  it  will  instantly  congeal.     Molecular 


THE  ROCKS  AND  THEIR  ORDER.  371 

rearrangements  are  thus  favoured  by  shocks,  and  the  con- 
sequences of  those  which  run  through  the  earth  are,  from 
a  chemical  point  of  view,  probably  important. 

The  reader  may  help  himself  to  understand  something 
of  the  complicated  problem  of  earth  tensions,  and  the 
corresponding  movements  of  the  rocks,  by  considering 
certain  homely  illustrations.  He  may  observe  how  the 
soil  cracks  as  it  shrinks  in  times  of  drought,  the  openings 
closing  when  it  rains.  In  a  similar  way  the  frozen  earth 
breaks  open,  sometimes  with  a  shock  which  is  often  counted 
as  an  earthquake.  Again,  the  ashes  in  a  sifter  or  the  gravel 
on  a  sieve  show  how  each  shaking  may  relieve  certain  ten- 
sions established  by  gravity,  while  they  create  others  which 
are  in  turn  to  be  released  by  the  next  shock.  An  ordi- 
nary dwelling  house  sways  and  strains  with  the  alterna- 
tions of  temperature  and  moisture  to  which  it  is  subjected 
in  the  round  of  climatal  alterations.  Now  and  then  we 
note  the  movements  in  a  cracking  sound,  but  by  far  the 
greater  part  of  them  escape  observation. 

With  this  sketch  of  the  mechanism  of  earthquake 
shocks  we  now  turn  to  consider  their  effects  upon  the  sur- 
face of  the  earth.  From  a  geological  point  of  view,  the 
most  important  effect  of  earthquake  shocks  is  found  in 
the  movement  of  rock  masses  down  steep  slopes,  which  is 
induced  by  the  shaking.  Everywhere  on  the  land  the 
agents  of  decay  and  erosion  tend  -to  bring  heavy  masses 
into  position  where  gravitation  naturally  leads  to  their 
downfall,  but  where  they  may  remain  long  suspended, 
provided  they  are  not  disturbed.  Thus,  wherever  there 
are  high  and  steep  cliffs,  great  falls  of  rock  are  likely  to 
occur  when  the  earthquake  movements  traverse  the  under 
earth.  In  more  than  one  instance  observers,  so  placed 
that  they  commanded  a  view  of  distant  mountains,  have 
noticed  the  downfall  of  precipices  in  the  path  of  the  shock 
before  the  trembling  affected  the  ground  on  which  they 
stood.  In  the  famous  earthquake  of  1783,  which  devas- 
tated southern  Italy,  the  Prince  of  Scylla  persuaded  his 


372  OUTLINES  OF  THE  EARTH'S  HISTORY. 

people  to  take  refuge  in  their  boats,  hoping  that  thej' 
might  thereby  escape  tlie  destruction  which  threatened 
them  on  the  land.  No  sooner  were  the  unhappy  folk  on  the 
water  than  the  fall  of  neighbouring  cliffs  near  the  sea 
produced  a  great  wave,  which  overwhelmed  the  vessels. 

Where  the  soil  lies  upon  steep  slopes,  in  positions  in 
which  it  has  accumulated  during  ages  of  tranquillity,  a 
great  shock  is  likely  to  send  it  down  into  the  valleys  in 
vast  landslides.  Thus,  in  the  earthquake  of  1692,  the 
Blue  Mountains  of  Jamaica  were  so  violently  shaken  that 
the  soil  and  the  forests  which  stood  on  it  were  precipi- 
tated into  the  river  beds,  so  that  many  tree-clad  summits 
became  fields  of  bare  rock.  The  effect  of  this  action  is 
immensely  to  increase  the  amount  of  detritus  which  the 
streams  convey  to  the  sea.  After  the  great  Jamaica  shock, 
above  noted,  the  rivers  for  a  while  ceased  to  flow,  their 
waters  being  stored  in  the  masses  of  loose  material.  Then 
for  weeks  they  poured  forth  torrents  of  mud  and  the  debris 
of  vegetation — materials  which  had  to  be  swept  away  as 
the  streams  formed  new  channels. 

In  all  regions  where  earthquake  movements  are  fre- 
quent, and  the  shock  of  considerable  violence,  the  trained 
observer  notes  that  the  surfaces  of  bare  rock  are  singularly 
extensive,  the  fact  being  that  many  of  these  areas,  where 
the  slope  lies  at  angles  of  from  ten  to  thirty  degrees, 
which  in  an  unshaken  region  would  be  thickly  soil-covered, 
are  deprived  of  the  coating  by  the  downward  movement 
of  the  waste  which  the  disturbances  bring  about.  A  famil- 
iar example  of  this  action  may  be  had  by  watching  the 
workmen  engaged  in  sifting  sand,  by  casting  the  material 
on  a  sloping  grating.  The  work  could  not  be  done  but 
for  an  occasional  blow  applied  to  the  sifter.  An  arrange- 
ment for  such  a  jarring  motion  is  commonly  found  in 
various  ore-dressing  machines,  where  the  object  is  to  move 
fragments  of  matter  over  a  sloping  surface. 

Even  where  the  earth  is  so  level  that  an  earthquake 
shock  does  not  cause  a  sliding  motion  of  the  materials, 


THE  ROCKS  AKD  THEIR  ORDER.  373 

such  as  above  described,  other  consequences  of  the  shak- 
ing may  readily  be  noted.  As  the  motion  runs  through 
the  mass,  provided  the  movement  be  one  of  considerable 
violence,  crevices  several  feet  in  width,  and  sometimes 
having  the  length  of  miles,  are  often  formed.  In  most 
cases  these  fissures,  opened  by  one  pulsation  of  the  shock, 
are  likely  to  be  closed  by  the  return  movement,  which 
occurs  the  instant  thereafter.  The  consequences  of  this 
action  are  often  singular,  and  in  cases  constitute  the 
most  frightful  elements  of  a  shock  which  the  sufferer 
beholds.  In  the  great  earthquake  of  1811,  which  rav- 
aged the  section  of  the  Mississippi  Valley  between  the 
mouth  of  the  Ohio  and  Vicksburg,  these  crevices  were  so 
numerously  formed  that  the  pioneers  protected  themselves 
from  the  danger  of  being  caught  in  their  jaws  by  felling 
trees  so  that  they  lay  at  right  angles  to  the  direction  in 
which  the  rents  extended,  building  on  these  timbers  plat- 
forms to  support  their  temporary  dwelling  places.  The 
records  of  earthquakes  supply  many  instances  in  which 
people  have  been  caught  in  these  earth  fissures,  and  in  a 
single  case  it  is  recorded  that  a  man  who  disappeared  into 
the  cavity  was  in  a  moment  cast  forth  in  the  rush  of  waters 
which  in  this,  as  in  many  other  cases,  spouts  forth  as  the 
walls  of  the  opening  come  together. 

Sometimes  these  rents  are  attended  by  a  dislocation, 
which  brings  the  earth  on  one  side  much  higher  than  on 
the  other.  The  step  thus  produced  may  be  many  miles 
in  length,  and  may  have  a  height  of  twenty  feet  or  more. 
It  needs  no  argument  to  show  that  we  have  here  the  top 
of  a  fault  such  as  produced  the  shock,  or  it  may  be  one 
of  a  secondary  nature,  such  as  any  earthquake  is  likely 
to  bring  about  in  the  strata  which  it  traverses.  In  cer- 
tain cases  two  faults  conjoin  their  action,  so  that  a  portion 
of  the  surface  disappears  beneath  the  earth,  entombing 
whatever  may  have  stood  on  the  vanished  site.  Thus  in 
the  great  shock  known  as  that  of  Lisbon,  which  occurred  in 
1755,  the  stone  quay  along  the  harbour,  where  many  thou- 


37i 


OUTLINES  OF  THE  EARTH'S  HISTORY. 


sand  people  had  sought  refuge  from  the  falling  buildings 
of  the  city,  suddenly  sank  down  with  the  multitude,  and 
the  waters  closed  over  it;  no  trace  of  the  people  or  of  the 
structure  was  to  be  found  after  the  shock  was  over.  There 
is  a  story  to  the  effect  that  during  the  same  earthquake 
an  Arab  village  in  northern  Africa  sank  down,  the  earth 
on  either  side  closing  over  it,  so  that  no.  trace  of  the 
habitations  remained.  In  both  these  instances  the  catas- 
trophes are  best  explained  by  the  diagram. 

In    the    earthquake 

— ^  lie    ^— 


B 


h  +  c 


of  1811  the  alluvial 
plains  on  either  side  of 
the  Mississippi  at  many 
points  sank  down  so 
that  arable  land  was 
converted  into  lakes; 
the  area  of  these  depres- 
sions probably  amount- 
ed to  some  hundred 
square  miles.  The 
writer,  on  examining 
these  sunken  lands, 
found  that  the  subsi- 
dences had  occurred 
where  the  old  moats  or 
abandoned  channels  of 
the  great  river  had  been 
filled  in  with  a  mixture 
of  decaying  timber  and 
river  silt.  When  vio- 
lently shaken,  this  loose-textured  debris  naturally  settled 
down,  so  that  it  formed  a  basin  occupied  by  a  crescent- 
shaped  lake.  The  same  process  of  settling  plentifully  goes 
on  wherever  the  rocks  are  still  in  an  uncemented 
state.  The  result  is  often  the  production  of  changes 
which  lead  to  the  expulsion  of  gases.  Thus,  in  the 
Charleston  earthquake  of  1883,  the  surface  over  an  area 


6' 

Fig.  21. — Diagram  showing  how  a  por- 
tion of  the  earth's  surface  may  be 
sunk  by  faulting.  Fig.  A  shows 
the  original  position  ;  B,  the  posi- 
tion after  faulting ;  b  b'  and  c  c' 
the  planes  of  the  faults ;  the  arrows 
the  direction  of  the  movement. 


THE  ROCKS  AND  THEIR  ORDER.  375 

of  many  hundred  square  miles  was  pitted  with  small 
craters,  formed  by  the  uprush  of  water  impelled  by  its 
contained  gases.  These  little  water  volcanoes — for  such 
we  may  call  them — sometimes  occur  to  the  number  of  a 
dozen  or  more  on  each  acre  of  ground  in  the  violently 
shaken  district.  They  indicate  one  result  of  the  physical 
and  chemical  alterations  which  earthquake  shocks  bring 
about.  As  earthquakes  increase  in  violence  their  effect 
upon  the  soil  becomes  continually  greater,  until  in  the 
most  violent  shocks  all  the  loose  materials  on  the  surface 
of  the  earth  may  be  so  shaken  about  as  to  destroy  even 
the  boundaries  of  fields.  After  the  famous  earthquake  of 
Eiobamba,  which  occurred  on  the  west  coast  of  South 
America  in  1797,  the  people  of  the  district  in  which  the 
town  of  that  name  was  situated  were  forced  to  redivide 
their  land,  the  original  boundaries  having  disappeared. 
Fortunately,  shocks  of  this  description  are  exceedingly 
rare.    They  occur  in  only  a  few  parts  of  the  world. 

Certain  effects  of  earthquakes  where  the  shock  emerges 
beneath  the  sea  have  been  stated  in  the  account  of  volcanic 
eruptions  (see  page  299).  We  may  therefore  note  here 
only  certain  of  the  more  general  facts.  While  passing 
through  the  deep  seas,  this  wave  may  have  a  height  of 
not  more  than  two  or  three  feet  and  a  width  of  some 
score  miles.  As  it  rolls  in  upon  the  shore  the  front  of 
the  undulation  is  retarded  by  the  friction  of  the  bottom  in 
such  a  measure  that  its  speed  is  diminished,  while  the 
following  part  of  the  waves,  being  less  checked,  crowds 
up  toward  this  forward  part.  The  result  is,  that  the  surge 
mounts  ever  higher  and  higher  as  it  draws  near  the  shore, 
upon  which  it  may  roll  as  a  vast  wave  having  the  height 
of  fifty  feet  or  more  and  a  width  quite  unparalleled  by 
any  wave  produced  from  wind  action.  Waves  of  this  de- 
scription are  most  common  in  the  Pacific  Ocean.  Al- 
though but  occasional,  the  damage  which  they  may  in- 
flict is  very  great.  As  the  movement  approaches  the 
shore,  vessels,  however  well  anchored,  are  dragged  away 
25 


376  OUTLINES  OF  THE  EARTH'S  HISTORY. 

to  seaward  by  the  great  back  lash  of  the  wave,  a  phe- 
nomenon which  may  be  perceived  even  in  the  case  of  the 
ordinary  surf.  Thus  forced  to  seaward,  the  crews  of  the 
ships  may  find  their  vessels  drawn  out  for  the  distance  of 
some  miles,  until  they  come  near  the  face  of  the  ad- 
vancing billow.  This,  as  .it  approaches  the  shore,  straight- 
ens up  to  the  wall-fronted  form,  and  then  topples  upon 
the  land.  Those  vessels  which  are  not  at  once  crushed 
down  by  the  blow  are  generally  hurled  far  inland  by  the 
rush  of  waters.  In  the  great  Jamaica  earthquake  of 
1692  a  British  man-of-war  was  borne  over  the  tops  of  cer- 
tain warehouses  and  deposited  at  a  distance  from  the  shore. 

Owing  to  the  fact  that  water  is  a  highly  elastic  mate- 
rial, the  shocks  transmitted  to  it  from  the  bottom  are 
sent  onward  with  their  energy  but  little  diminished.  While 
the  impulse  is  very  violent,  these  oscillations  may  prove 
damaging  to  shipping.  The  log-books  of  mariners  abound 
in  stories  of  how  vessels  were  dismasted  or  otherwise 
badly  shaken  by  a  sudden  blow  received  in  the  midst 
of  a  quiet  sea.  The  impression  commonly  conveyed  to 
the  sailors  is  that  the  craft  has  struck  upon  a  rock.  The 
explanation  is  that  an  earthquake  jar,  in  traversing  the 
water,  has  delivered  its  blow  to  the  ship.  As  the  speed 
of  this  jarring  movement  is  very  much  greater  than  that 
of  any  ordinary  wave,  the  blow  which  it  may  strike  may 
be  most  destructive.  There  seems,  indeed,  little  reason 
to  doubt  that  a  portion  of  the  vessels  which  are  ever  dis- 
appearing in  the  wilderness  of  the  ocean  are  lost  by  the 
crushing  effect  of  these  quakings  which  pass  through  the 
waters  of  the  deep. 

We  have  already  spoken  of  the  earthquake  shock  as 
an  oscillation.  It  is  a  quality  of  all  bodies  which  oscil- 
late under  the  influence  of  a  blow,  such  as  originates  in 
earthquake  shocks,  to  swing  to  and  fro,  after  the  manner 
of  the  metal  in  a  bell  or  a  tuning  fork,  in  a  succession 
of  movements,  each  less  than  the  preceding,  until  the 
impulse  is  worn  out,  or  rather,  we  should  in  strict  sense 


THE   ROCKS  AND  THEIR  ORDER.  377 

say,  changed  to  other  forms  of  energy.  The  result  is,  that 
even  in  the  slightest  earthquake  shock  the  earth  moves 
not  once  to  and  fro,  but  very  many  times.  In  a  consider- 
able shock  the  successive  diminishing  swingings  amount 
to  dozens  before  they  become  so  slight  as  to  elude  per- 
ception. Although  the  first  swaying  is  the  strongest,  and 
generally  the  most  destructive,  the  quick  to-and-fro  mo- 
tions are  apt  to  continue  and  to  complete  the  devastation 
which  the  first  brings  about.  The  vibrations  due  to  any 
one  shock  take  place  with  great  rapidity.  They  may, 
indeed,  be  compared  to  those  movements  which  we  per- 
ceive in  the  margin  of  a  large  bell  w^lien  it  has  received 
a  heavy  blow  from  the  clapper.  The  reader  has  perhaps 
seen  that  for  a  moment  the  rim  of  the  bell  vibrates  with 
such  rapidity  that  it  has  a  misty  look — that  is,  the  motions 
elude  the  sight.  It  is  easy  to  see  that  a  shaking  of  this 
kind  is  particularly  calculated  to  disrupt  any  bodies  which 
stand  free  in  the  air  and  are  supported  only  at  their  base. 

In  what  we  may  call  the  natural  architecture  of  the 
earth,  the  pinnacles  and  obelisks,  such  as  are  formed  in 
many  high  countries,  the  effect  of  these  shakings  is  de- 
structive, and,  as  we  have  seen,  even  the  firmer-placed 
objects,  such  as  the  strong-walled  cliffs  and  steep  slopes 
of  earth,  break  down  under  the  assaults.  It  is  therefore 
no  matter  of  surprise  that  the  buildings  which  man  erects, 
where  they  are  composed  of  masonry,  suffer  greatly  from 
these  tremblings.  In  almost  all  cases  human  edifices  are 
constructed  without  regard  to  other  problems  of  strength 
than  those  which  may  be  measured  by  their  weight  and 
the  resistance  to  fracture  from  gravitation  alone.  They 
are  not  built  with  expectation  of  a  quaking,  but  of  a  firm- 
set  earth. 

The  damage  which  earthquakes  do  to  buildings  is  in 
most  cases  due  to  the  fact  that  they  sway  their  walls  out 
of  plumb,  so  that  they  are  no  longer  in  position  to  sup- 
port the  weight  which  they  have  to  bear.  The  amount 
of  this  swaying  is  naturally  very  much  greater  them  that 


378  OUTLINES  OF  THE  EARTH'S  HISTORY. 

which  the  earth  itself  experiences  in  the  movement.  A 
building  of  any  height  with  its  walls  unsupported  by 
neighbouring  structures  may  find  its  roof  rocked  to  and 
fro  through  an  arc  which  has  a  length  of  feet,  while  its 
base  moves  only  through  a  length  of  inches.  The  reader 
may  see  an  example  of  this  nature  if  he  will  poise  a  thin 
book  or  a  bit  of  plank  a  foot  long  on  top  of  a  small  table; 
then  jarring  the  table  so  that  it  swings  through  a  distance 
of  say  a  quarter  of  an  inch,  he  will  see  that  the  columnar 
object  swings  at  its  top  through  a  much  greater  distance, 
and  is  pretty  sure  to  be  overturned. 

Where  a  building  carries  a  load  in  its  upper  parts, 
such  as  may  be  afforded  by  its  heavy  roof  or  the  stores 
which  it  contains,  the  effect  of  an  earthquake  shock  such 
as  carries  the  earth  to  and  fro  becomes  much  more  de- 
structive than  it  might  otherwise  be.  This  weight  lags 
behind  when  the  earth  slips  forward  in  the  first  move- 
ment of  the  oscillation,  with  the  effect  tliat  the  walls  of 
the  building  are  pretty  sure  to  be  thrust  so  far  beyond 
the  perpendicular  that  they  give  way  and  are  carried  down 
by  the  weight  which  they  bore.  It  has  often  been  re- 
marked in  earthquake  shocks  that  tall  columns,  even 
where  composed  of  many  blocks,  survive  a  shock  which 
overturns  lower  buildings  where  thin  walls  support  sev- 
eral floors,  on  each  of  which  is  accumulated  a  consider- 
able amount  of  weight.  In  the  case  of  the  column,  the 
strains  are  even,  and  the  whole  structure  may  rock  to  and 
fro  without  toppling  over.  As  the  energy  of  the  undula- 
tions diminish,  it  gradually  regains  the  quiet  state  without 
damage.  In  the  ordinary  edifice  the  irregular  disposition 
of  the  weight  does  not  permit  the  uniform  movement 
which  may  insure  safety.  Thus,  if  the  city  of  Washington 
should  ever  be  violently  shaken,  the  great  obelisk,  not- 
withstanding that  it  is  five  hundred  feet  high,  may  sur- 
vive a  disturbance  which  would  wreck  the  lower  and 
more  massive  edifices  which  lie  about  it. 

Where,  as  is  fortunately  rarely  the  case^  the  great  shock 


THE  ROCKS  AND  THEIR  ORDER.  379 

comes  to  the  earth  in  a  vertical  direction,  the  effect  upon 
all  movable  objects  is  in  the  highest  measure  disastrous. 
In  such  a  case  buildings  are  crushed  as  if  by  the  stroke 
of  a  giant^s  hand.  The  roofs  and  floors  are  at  one  stroke 
thrown  to  the  foundations,  and  all  the  parts  of  the  walls 
which  are  not  supported  by  strong  masonry  continuous 
from  top  to  bottom  are  broken  to  pieces.  In  such  cases  it 
has  been  remarked  that  the  bodies  of  men  are  often  thrown 
considerable  distances.  It  is  asserted,  indeed,  that  in  the 
Eiobamba  shock  they  were  cast  upward  to  the  height  of 
more  than  ninety  feet.  It  is  related  that  the  sole  sur- 
vivor of  a  congregation  which  had  hastened  at  the  outset 
of  the  disturbance  into  a  church  was  thrown  by  the  great- 
est and  most  destructive  shock  upward  and  through  a 
window  the  base  of  which  was  at  the  height  of  more  than 
twenty  feet  from  the  ground. 

It  is  readily  understood  that  an  earthquake  shock 
may  enter  a  building  in  any  direction  between  the  verti- 
cal and  the  horizontal.  As  the  movement  exhausts  itself 
in  passing  from  the  place  of  its  origin,  the  horizontal 
shocks  are  usually  of  least  energy.  Those  which  are  ac- 
curately vertical  are  only  experienced  where  the  edifices 
are  placed  immediately  over  the  point  where  the  motion 
originates.  It  follows,  therefore,  that  the  destructive  work 
of  earthquakes  is  mainly  performed  in  that  part  of  the 
field  where  the  motion  is,  as  regards  its  direction,  be- 
tween the  vertical  and  the  horizontal — a  position  in  which 
the  edifice  is  likely  to  receive  at  once  the  destructive  effect 
arising  from  the  sharp  upward  thrust  of  the  vertical 
movement  and  the  oscillating  action  of  that  which  is  in 
a  horizontal  direction.  Against  strains  of  this  descrip- 
tion, where  the  movements  have  an  amplitude  of  more 
than  a  few  inches,  no  ordinary  masonry  edifice  can  be 
made  perfectly  safe;  the  only  tolerable  security  is  at- 
tained where  the  building  is  of  well-framed  timber,  which 
by  its  elasticity  permits  a  good  deal  of  motion  without 
destructive  consequences.     Even  such  buildings,  however. 


380         OUTLINES  OF  THE  EARTH'S  HISTORY. 

those  of  the  strongest  type,  may  be  ruined  by  the  greater 
earthquakes.  Thus,  in  the  Mississippi  Valley  earthquake 
of  1811,  the  log  huts  of  the  frontiersmen,  which  are  about 
as  strong  as  any  buildings  can  be  made,  were  shaken  to 
pieces  by  the  sharp  and  reiterated  shocks. 

It  is  by  no  means  surprising  to  find  that  the  style 
of  architecture  adopted  in  earthquake  countries  differs 
from  that  which  is  developed  in  regions  where  the  earth 
is  firm-set.  The  people  generally  learn  that  where  their 
buildings  must  meet  the  trials  of  earthquakes  they  have 
to  be  low  and  strong,  framed  in  the  manner  of  fortifica- 
tions, to  withstand  the  assault  of  this  enemy.  We  observe 
that  Gothic  architecture,  where  a  great  weight  of  masonry 
is  carried  upon  slender  columns  and  walls  divided  by  tall 
windows,  though  it  became  the  dominant  style  in  the  rela- 
tively stable  lands  of  northern  Europe,  never  gained  a 
firm  foothold  in  those  regions  about  the  Mediterranean 
which  are  frequently  visited  by  severe  convulsions  of  the 
earth.  There  the  Grecian  or  the  Eomanesque  styles, 
which  are  of  a  much  more  massive  type,  retain  their  places 
and  are  the  fashions  to  the  present  day.  Even  this  man- 
ner of  building,  though  affording  a  certain  security  against 
slight  tremblings,  is  not  safe  in  the  greater  shocks.  Again 
and  again  large  areas  in  southern  Italy  have  been  almost 
swept  of  their  buildings  by  the  destructive  movements 
which  occur  in  that  realm.  The  only  people  who  have 
systematically  adapted  their  architectural  methods  to 
earthquake  strains  are  the  Japanese,  who  in  certain  dis- 
tricts where  such  risks  are  to  be  encountered  construct 
their  dwellings  of  wood,  and  place  them  upon  rollers,  so 
that  they  may  readily  move  to  and  fro  as  the  shock  passes 
beneath  them.  In  a  measure  the  people  of  San  Fran- 
cisco have  also  provided  against  this  danger  by  avoiding 
dangerous  weights  in  the  upper  parts  of  their  buildings,  as 
well  as  the  excessive  height  to  which  these  structures  are 
lifted  in  some  of  our  American  towns. 

Earthquakes  of  sensible  energy  appear  to  be  limited 


THE  ROCKS  AND  THEIR  ORDER.  381 

to  particular  parts  of  the  earth's  crust.  The  regions,  in- 
deed, where  within  the  period  of  human  history  shocks 
of  devastating  energy  have  occurred  do  not  include  more 
than  one  fifteenth  part  of  the  earth's  surface.  There  is 
a  common  notion  that  these  movements  are  most  apt  to 
happen  in  volcanic  regions.  It  is,  indeed,  true  that  sensi- 
ble shocks  commonly  attend  the  explosions  from  great 
craters,  but  the  records  clearly  show  that  these  movements 
are  very  rarely  of  destructive  energy.  Thus  in  the  regions 
about  the  base  of  Vesuvius  and  of  ^tna,  the  two  vol- 
canoes of  which  most  is  known,  the  shocks  have  never 
been  productive  of  extensive  disaster.  In  fact,  the  reit- 
erated slight  jarrings  which  attend  volcanic  action  appear 
to  prevent  the  formation  of  those  great  and  slowly  accu- 
mulated strains  which  in  tlieir  discharge  produce  the  most 
violent  tremblings  of  the  earth.  The  greatest  and  most 
continuous  earthquake  disturbances  of  history — that  be- 
fore noted  in  the  early  days  of  this  century,  in  the  Missis- 
sippi Valley,  where  shocks  of  considerable  violence  con- 
tinued for  two  years — came  about  in  a  field  very  far  re- 
moved from  active  volcanoes.  So,  too,  the  disturbances 
beneath  the  Atlantic  floor  which  originated  the  shocks 
that  led  to  the  destruction  of  Lisbon,  and  many  other 
similar  though  less  violent  movements,  are  developed  in 
a  field  apparently  remote  from  living  volcanoes.  Eastern 
New  England,  which  has  been  the  seat  of  several  consid- 
erable earthquakes,  is  about  as  far  away  from  active  vents 
as  any  place  on  the  habitable  globe.  We  may  therefore 
conclude  that,  while  volcanoes  necessarily  produce  shocks 
resulting  from  the  discharge  of  their  gases  and  the  in- 
trusion of  lava  into  the  dikes  which  are  formed  about  them, 
the  greater  part  of  the  important  shocks  are  in  no  wise 
connected  with  volcanic  explosions. 

With  the  exception  of  the  earthquake  in  the  Missis- 
sippi Valley,  all  the  great  shocks  of  which  we  have  a 
record  have  occurred  in  or  near  regions  where  the  rocks 
have   been   extensively   disturbed   by   mountain-building 


382  OUTLINES  OF  THE  EARTH'S  HISTORY. 

forces,  and  where  the  indications  lead  us  to  believe  that 
dislocations  of  strata,  such  as  are  competent  to  rive  the  beds 
asunder,  may  still  be  in  progress.  This,  taken  in  con- 
nection with  the  fact  that  many  of  these  shocks  are  at- 
tended by  the  formation  of  fault  planes,  which  appear 
on  the  surface,  lead  us  to  the  conclusion  that  earthquakes 
of  the  stronger  kind  are  generally  formed  by  the  riving 
of  fissures,  which  may  or  may  not  be  developed  upward 
to  the  surface.  This  view  is  supported  by  many  careful 
observations  on  the  effect  which  certain  great  earthquakes 
have  exercised  on  the  buildings  which  they  have  rav- 
aged. The  distinguished  observer,  Mr.  Charles  Mallet,  who 
visited  the  seat  of  the  earthquake  which,  in  1854,  oc- 
curred in  the  province  of  Calabria  in  Italy,  with  great 
labour  and  skill  determined  the  direction  in  which  the 
shock  moved  through  some  hundreds  of  edifices  on  which 
it  left  the  marks  of  its  passage.  Platting  these  lines  of 
motion,  he  found  that  they  were  all  referred  to  a  vertical 
plane  lying  at  the  depth  of  some  miles  beneath  the  sur- 
face, and  extending  for  a  great  distance  in  a  north  and 
south  direction.  This  method  of  inquiry  has  been  applied 
to  other  fields,  with  the  result  that  in  the  case  of  all  the 
instances  which  have  been  subjected  to  this  inquiry  the 
seat  of  the  shock  has  been  traced  to  such  a  plane,  which 
can  best  be  accounted  for  by  the  supposition  of  a  fault. 
The  method  pursued  by  Mr.  Mallet  in  his  studies  of 
the  origin  of  earthquakes,  and  by  those  who  have  con- 
tinued his  inquiry,  may  be  briefly  indicated  as  foUow^s: 
Examining  disrupted  buildings,  it  is  easy  to  determine 
those  which  have  been  wrecked  by  a  shock  that  emerged 
from  the  earth  in  a  vertical  direction.  In  these  cases, 
though  tall  walls  may  remain  standing,  the  roofs  and 
floors  are  thrown  into  the  cellars.  With  a  dozen  such  in- 
stances the  plane  of  what  is  called  the  seismic  vertical 
is  established  (seismos  is  the  Greek  for  earthquake).  Then 
on  either  side  of  this  plane,  which  indicates  the  line  but 
not  the  depth  of  the  disturbance,  other  observations  may 


THE  ROCKS  AND  THEIR  ORDER.  383 

be  made  which  give  the  clew  to  the  depth.  Thus  a  build- 
ing may  be  found  where  the  northwest  corner  at  its 
upper  part  has  been  thrown  off.  Such  a  rupture  was  clearly 
caused  by  an  upward  but  oblique  movement,  which  in  the 
first  half  of  the  oscillation  heaved  the  structure  upwardly 
into  the  northwest,  and  then  in  the  second  half,  or  rebound, 
drew  the  mass  of  the  building  away  from  the  unsupported 
corner,  allowing  that  part  of  the  masonry  to  fly  off  and 
fall  to  the  ground.  Constructing  a  line  at  right  angles 
to  the  plane  of  the  fracture,  it  will  be  found  to  intersect 
the  plane,  the  position  of  which  has  been  in  part  deter- 
mined by  finding  the  line  where  it  intersects  the  earth, 
or  the  seismic  vertical  before  noted.  Multiplying  such 
observations  on  either  side  of  the  last-mentioned  line, 
the  attitude  of  the  underground  parts  of  the  plane,  as 
well  as  the  depth  to  which  it  attained,  can  be  approxi- 
mately determined. 

It  is  worth  while  to  consider  the  extent  to  which 
earthquake  shocks  may  affect  the  general  quality  of  the 
people  who  dwell  in  countries  where  these  disturbances 
occur  with  such  frequency  and  violence  as  to  influence 
their  lives.  There  can  be  no  question  that  wherever 
earthquakes  occur  in  such  a  measure  as  to  produce  wide- 
spread terror,  where,  recurring  from  time  to  time,  they 
develop  in  men  a  sense  of  abiding  insecurity,  they  be- 
come potent  agents  of  degradation.  All  the  best  which 
men  do  in  creating  a  civilization  rests  upon  a  sense  of  con- 
fidence that  their  efforts  may  be  accumulated  from  year 
to  year,  and  that  even  after  death  the  work  of  each  man 
may  remain  as  a  heritage  to  his  kind.  It  is  likely,  indeed, 
that  in  certain  realms,  as  in  southern  Italy,  a  part  of 
the  failure  of  the  people  to  advance  in  culture  is  due  to 
their  long  experience  of  such  calamities,  and  the  natural 
expectation  that  they  will  from  time  to  time  recur.  In 
a  similar  way  the  Spanish  settlements  in  Central  and 
South  America,  which  lie  mostly  in  lands  that  are  sub- 
ject  to   disastrous   shocks,   may   have   been   retarded  by 


384  OUTLINES  OF  THE  EARTH'S  HISTORY. 

the  despair,  as  well  as  the  loss  of  property  and  life,  which 
these  accidents  have  so  frequently  inflicted  upon  them. 
It  will  not  do,  however,  to  attribute  too  much  to  such 
terrestrial  influences.  By  far  the  most  important  ele- 
ment in  determining  the  destiny  of  a  people  is  to  be 
found  in  their  native  quality,  that  which  they  ow^e  to  their 
ancestors  of  distant  generations.  In  this  connection  it 
is  well  to  consider  the  history  of  the  Icelandic  people, 
where  a  small  folk  has  for  a  thousand  years  been  exposed 
to  a  range  and  severity  of  trials,  such  as  earthquakes,  vol- 
canic explosions,  and  dearth  of  harvests  may  produce,  and 
all  these  in  a  measure  that  few  if  any  other  countries 
experience.  Notwithstanding  these  misfortunes,  the  Ice- 
landers have  developed  and  maintained  a  civilization 
which  in  all  else,  except  its  material  results,  on  the  average 
transcends  that  which  has  been  won  by  any  other  folk 
in  modern  times.  If  a  people  have  the  determining  spirit 
which  leads  to  high  living,  they  can  successfully  face 
calamities  far  greater  than  those  which  earthquakes  in- 
flict. 

It  was  long  supposed  that  the  regions  where  earth- 
quakes are  not  noticeable  by  the  unaided  senses  were  ex- 
empt from  all  such  disturbances.  The  observations  which 
seismologists  have  made  in  recent  years  point  to  the  con- 
clusion that  no  part  of  the  earth's  surface  is  quite  ex- 
empt from  movements  which,  though  not  readily  per- 
ceived, can  be  made  visible  by  the  use  of  appropriate 
instruments.  With  an  apparatus  known  as  the  horizontal 
pendulum  it  is  possible  to  observe  vibrations  which  do 
not  exceed  in  amplitude  the  hundredth  part  of  an  inch. 
This  mechanism  consists  essentially  of  a  slender  bar  sup- 
ported near  one  end  by  two  wires,  one  from  above,  the 
other  from  below.  It  may  readily  be  conceived  that  any 
measurable  movement  will  cause  the  longer  end  of  the 
rod  to  sway  through  a  considerable  arc.  Wherever  such 
a  pendulum  has  been  carefully  observed  in  any  district, 
it  has  been  found  that  it  indicates  the  occurrence  of  slight 


THE  HOCKS  AND  THEIR  ORDER.  3S5 

tremors.  Even  certain  changes  of  the  barometer,  which 
alter  the  weight  of  the  atmosphere  that  rests  upon  tlie 
earth  to  the  amount  indicated  by  an  inch  in  the  height 
of  the  mercury  column,  appears  in  all  cases  to  create  such 
tremors.  Many  of  these  slight  shocks  may  be  due  to  the 
effect  of  more  violent  quakings,  which  have  run  perhaps 
for  thousands  of  miles  from  their  point  of  origin,  and 
have  thus  been  reduced  in  the  amplitude  of  their  move- 
ment. Others  are  probably  due  to  the  slight  motion 
brought  about  through  the  chemical  changes  of  the  rocks, 
which  are  continuously  going  on.  The  ease  with  which 
even  small  motions  are  carried  to  a  great  distance  may 
be  judged  by  the  fact  that  when  the  ground  is  frozen  the 
horizontal  pendulum  will  indicate  the  jarring  due  to  a 
railway  train  at  the  distance  of  a  mile  or  more  from  the 
track. 

In  connection  with  the  earth  jarring,  it  would  be 
well  to  note  the  occurrence  of  another,  though  physically 
different,  kind  of  movement,  which  we  may  term  earth 
swayi-ngs,  or  massive  movements,  which  slowly  dislocate 
the  vertical,  and  doubtless  also  the  horizontal,  position 
of  points  upon  its  surface.  It  has  more  than  once  been 
remarked  that  in  mountain  countries,  where  accurate 
sights  have  been  taken,  the  heights  of  points  between 
the  extremities  of  a  long  line  appear  somewhat  to  vary 
in  the  course  of  a  term  of  years.  Thus  at  a  place  in  the 
Apennines,  where  two  buildings  separated  by  some  miles 
of  distance  are  commonly  intervisible  over  the  crest  of  a 
neighbouring  peak,  it  has  happened  that  a  change  of  level 
of  some  one  of  the  points  has  made  it  impossible  to  see 
the  one  edifice  from  the  other.  Knowing  as  we  do  that 
the  line  of  the  seacoast  is  ever-changing,  uprising  taking 
place  at  some  points  and  down-sinking  at  others,  it  seems 
not  unlikely  that  these  irregular  swayings  are  of  very 
common  occurrence.  Moreover,  astronomers  are  begin- 
ning to  remark  the  fact  that  their  observatories  appear 
not  to  remain  permanently  in  the  same   position — that 


386  OUTLINES  OP  THE  EARTH'S  HISTORY. 

is,  they  do  not  have  exactly  the  same  latitude  and  longi- 
tude. Certain  of  these  changes  have  recently  heen  ex- 
plained by  the  discovery  of  a  new  and  hitherto  unnoted 
movement  of  the  polar  axis.  It  is  not  improbable,  how- 
ever, that  the  irregular  swaying  of  the  earth's  crust,  due 
to  the  folding  of  strata  and  to  the  alterations  in  the  vol- 
ume of  rocks  which  are  continually  going  on,  may  have 
some  share  in  bringing  about  these  dislocations. 

Measured  by  the  destruction  which  was  wrought  to 
the  interests  of  man,  earthquakes  deserve  to  be  reckoned 
among  the  direst  calamities  of  Nature.  Since  the  dawn 
of  history  the  records  show  us  that  the  destruction  of 
life  which  is  to  be  attributed  to  them  is  to  be  counted 
by  the  millions.  A  catalogue  of  the  loss  of  life  in  the 
accidents  of  this  description  which  have  occurred  during 
the  Christian  era  has  led  the  writer  to  suppose  that  prob- 
ably over  two  million  persons  have  perished  from  these 
shocks  in  the  last  nineteen  centuries.  Nevertheless,  as 
compared  with  other  agents  of  destruction,  such  as  pre- 
ventable disease,  war,  or  famine,  the  loss  which  has.  been 
inflicted  by  earth  movements  is  really  trifling,  and  almost 
all  of  it  is  due  to  an  obstinate  carelessness  in  the  con- 
struction of  buildings  without  reference  to  the  risks  which 
are  known  to  exist  in  earthquake-ridden  countries. 

Although  all  our  exact  knowledge  concerning  the  dis- 
tribution of  earthquakes  is  limited  to  the  imperfect  records 
of  two  or  three  thousand  years,  it  is  commonly  possible 
to  measure  in  a  general  way  the  liability  to  such  accidents 
which  may  exist  in  any  country  by  a  careful  study  of  the 
details  of  its  topography.  In  almost  every  large  area  the 
process  of  erosion  naturally  leaves  quantities  of  rock, 
either  in  the  form  of  detached  columns  or  as  detrital 
accumulations  deposited  on  steep  slopes.  These  features 
are  of  relatively  slow  formation,  and  it  is  often  possible 
to  determine  that  they  have  been  in  their  positions  for 
a  time  which  is  to  be  measured  by  thousands  of  years. 
Thus,  on  inspecting  a  country  such  as  North  America, 


THE  ROCKS  AND  THEIR  ORDER.  337 

where  the  historic  records  cover  but  a  brief  time,  we  may 
on  inquiry  determine  which  portions  of  its  area  have  long 
been  exempt  from  powerful  shocks.  Where  natural  obe- 
lisks and  steep  taluses  abound — features  which  would  have 
disappeared  if  the  region  had  been  moved  by  great  shocks 
— we  may  be  sure  that  the  field  under  inspection  has  for 
a  great  period  been  exempt  from  powerful  shaking.  Judged 
by  this  standard,  we  may  safely  say  that  the  region  occu- 
pied by  the  Appalachian  Mountains  has  been  exempt  from 
serious  trouble.  So,  too,  the  section  of  the  Cordilleras 
lying  to  the  east  of  what  is  commonly  called  the  Great 
Basin,  between  the  Rocky  Mountains  and  the  Sierra 
Nevada,  has  also  enjoyed  a  long  reign  of  peace.  In  glaci- 
ated countries  the  record  is  naturally  less  clear  than  in 
those  parts  of  the  world  which  have  been  subjected  to 
long-continued,  slow  decay  of  the  rocks.  Nevertheless, 
in  those  fields  boulders  are  often  found  poised  in  position 
which  they  could  not  have  maintained  if  subjected  to 
violent  shaking.  Judged  by  this  evidence,  we  may  say 
that  a  large  part  of  the  northern  section  of  this  continent, 
particularly  the  area  about  the  Great  Lakes,  has  been  ex- 
empt from  considerable  shocks  since  the  glacier  passed 
away. 

The  shores  which  are  subject  to  the  visitations  of  the 
great  marine  waves,  caused  by  earthquake  shocks  occur- 
ring beneath  the  bottom  of  the  neighbouring  ocean,  are 
so  swept  by  those' violent  inundations  that  they  lose  many 
features  which  are  often  found  along  coasts  that  have 
been  exempted  from  such  visitations.  Thus  wherever 
we  find  extensive  and  delicately  moulded  duiies,  poised 
stones,  or  slender  pinnacled  rocks  along  a  coast,  we  may 
be  sure  that  since  these  features  were  formed  the  district 
has  not  been  swept  by  these  great  waves. 

Around  the  northern  Atlantic  we  almost  everywhere 
find  the  glacial  waste  here  and  there  accumulated  near 
the  margin  of  the  sea  in  the  complicated  sculptured  out- 
lines which  are  assumed  by  kame  sands  and  gravels.    From 


388 


OUTLINES  OF  THE  EARTH'S  HISTORY. 


Fig.  22. — Poised  rocks  indicating  a  long  exemption  from  strong  earth- 
quakes in  the  places  where  such  features  occur. 

a  study  of  these  features  just  above  the  level  of  high  tide, 
the  writer  has  become  convinced  that  the  North  Atlantic 
district  has  long  been  exempt  from  the  assaults  of  other 
waves  than  those  which  are  produced  during  heavy  storms, 


THE  ROCKS  AND  THEIR  ORDER.  389 

At  the  present  time  the  waves  formed  by  earthquakes 
appear  to  be  of  destructive  violence  only  on  the  west  coast 
of  South  America,  where  they  roll  in  from  a  region  of  the 
Pacific  lying  to  the  south  of  the  equator  and  a  few  hundred 
miles  from  the  shore  of  the  continent,  which  appears  to 
be  the  seat  of  exceedingly  violent  shocks.  A  similar  field 
occurs  in  the  Atlantic  between  the  Lesser  Antilles  and  the 
Spanish  peninsula,  but  no  great  waves  have  come  thence 
since  the  time  of  the  Lisbon  earthquake.  The  basin  of 
the  Caribbean  and  the  region  about  Java  appear  to  be  also 
fields  where  these  disturbances  may  be  expected,  though 
in  each  but  one  wave  of  this  nature  has  been  recorded. 
Therefore  we  may  regard  these  secondary  results  of  a  sub- 
marine earthquake  as  seldom  phenomena. 

DuKATiON  OF  Geological  Time. 

Although  it  is  beyond  the  power  of  man  to  conceive 
any  such  lapses  of  time  as  have  taken  place  in  the  history 
of  this  earth,  it  is  interesting,  and  in  certain  ways  profit- 
able, to  determine  as  near  as  possible  in  the  measure  of 
years  the  duration  of  the  events  which  are  recorded  in 
the  rocks.  Some  astronomers,  basing  their  conclusions 
on  the  heat-containing  power  of  matter,  and  on  the  rate 
at  which  energy  in  this  form  flows  from  the  sun,  have 
come  to  the  conclusion  that  our  planet  could  not  have  been 
in  independent  existence  for  more  than  about  twenty 
million  years.  The  geologist,  however,  resting  his  con- 
clusions on  the  records  which  are  the  subject  of  his  in- 
quiry, comes  on  many  different  lines  to  an  opinion  which 
traverses  that  entertained  by  some  distinguished  astrono- 
mers. The  ways  in  which  the  student  of  the  earth  arrives 
at  this  opinion  will  now  be  set  forth. 

By  noting  the  amount  of  sediment  carried  forth  to 
the  sea  by  the  rivers,  the  geologist  finds  that  the  lands  of 
the  earth — ^those,  at  least,  which  are  protected  by  their 
natural  envelopes  of  vegetation — are  wearing  down  at  a 


390  OUTLINES  OF  THE  EARTH'S  HISTORY. 

rate  which  pretty  certainly  does  not  exceed  one  foot  in 
about  five  thousand  years,  or  two  hundred  feet  in  a  mil- 
lion years.  Discovering  at  many  places  on  the  earth's 
surface  deposits  which  originally  had  a  thickness  of  five 
thousand  feet  or  more,  which  have  been  worn  down  to  the 
depths  of  thousands  of  feet  in  a  single  rather  brief  sec- 
tion of  geological  time,  the  student  readily  finds  himself 
prepared  to  claim  that  a  period  of  from  five  to  ten  million 
years  has  often  been  required  for  the  accomplishment  of 
but  a  very  small  part  of  the  changes  which  he  knows  to 
have  occurred  on  this  earth. 

As  the  geologist  follows  down  through  the  sections  of 
the  stratified  rocks,  and  from  the  remains  of  strata  deter- 
mines the  erosion  which  has  borne  away  the  greater  part 
of  the  thick  deposits  which  have  been  exposed  to  erosion, 
he  comes  upon  one  of  those  breaks  in  the  succession,  or 
encounters  what  is  called  an  unconformity,  as  when  hori- 
zontal strata  lie  against  those  which  are  tilted.  In  many 
cases  he  may  observe  that  at  this  time  there  was  a  great 
interval  unrepresented  by  deposits  at  the  place  where  his 
observations  are  made,  yet  a  great  lapse  of  time  is  indi- 
cated by  the  fact  that  a  large  amount  of  erosion  took  place 
in  the  interval  between  the  two  sets  of  beds. 

Putting  together  the  bits  of  record,  and  assuming  that 
the  rate  of  erosion  accomplished  by  the  agents  which 
operate  on  the  land  has  always  been  about  the  same, 
the  geologist  comes  to  the  conclusion  that  the  section  of 
the  rocks  from  the  present  day  to  the  lowest  strata  of  the 
Laurentian  represents  in  the  time  required  for  their  forma- 
tion not  less  than  a  hundred  million  years;  more  likely 
twice  that  duration.  To  this  argument  objection  is  made 
by  some  naturalists  that  the  agents  of  erosion  may  have 
been  more  active  in  the  past  than  they  are  at  present. 
They  suggest  that  the  rainfall  may  have  been  much 
greater  or  the  tides  higher  than  they  now  are.  Granting 
all  that  can  be  claimed  on  this  score,  we  note  the  fact 
that  the  rate  of  erosion  evidently  does  not  increase  in 


THE  ROCKS  AND  THEIR  ORDER.  391 

anything  like  a  proportionate  way  with  the  amount  of 
rainfall.  Where  a  country  is  protected  by  its  natural 
coating  of  vegetation,  the  rain  is  delivered  to  the  streams 
without  making  any  considerable  assault  upon  the  sur- 
face of  the  earth,  however  large  the  fall  may  be.  More- 
over, the  tides  have  little  direct  cutting  power;  they  can 
only  remove  detritus  which  other  agents  have  brought  into 
a  condition  to  be  borne  away.  The  direct  cutting  power 
of  the  tidal  movement  does  not  seem  to  be  much  greater 
in  the  Bay  of  Fundy,  where  the  maximum  height  of  the 
waves  amounts  to  fifty  feet,  than  on  the  southern  coast 
of  Massachusetts,  where  the  range  is  not  more  than  five. 
So  far  as  the  observer  can  judge,  the  climatal  conditions 
and  the  other  influences  which  affect  the  wear  of  rocks 
have  not  greatly  varied  in  the  past  from  what  they  are 
at  the  present  day.  Now  and  then  there  have  been  periods 
of  excessive  erosion;  again,  ages  in  which  large  fields 
were  in  the  conditions  of  exceeding  drought.  It  is,  how- 
ever, a  fair  presumption  that  these  periods  in  a  way  bal- 
ance each  other,  and  that  the  average  state  was  much  like 
that  which  we  find  at  present. 

If  after  studying  the  erosive  phenomena  exhibited  in 
the  structure  of  the  earth  the  student  takes  up  the  study 
of  the  accumulations  of  strata,  and  endeavours  to  deter- 
mine the  time  required  for  the  laying  down  of  the  sedi- 
ments, he  finds  similar  evidence  of  the  earth's  great  an- 
tiquity. Although  the  process  of  deposition,  which  has 
given  us  the  rocks  visible  in  the  land  masses,  has  been 
very  much  interrupted,  the  section  which  is  made  by 
grouping  the  observations  made  in  various  fields  shows 
that  something  like  a  maximum  thickness  of  a  hundred 
and  fifty  thousand  feet  of  beds  has  been  accumulated  in 
that  part  of  geologic  time  during  which  strata  were  being 
laid  down  in  the  fields  that  are  subjected  to  our  study. 
Although  in  these  rocks  there  are  many  sets  of  beds  which 
were  rapidly  formed,  the  greater  part  of  them  have  been 
accumulated  with  exceeding  slowness.  Many  fine  shales, 
26 


392  OUTLINES  OF  THE  EARTH'S  HISTORY. 

such  as  those  which  plentifully  occur  in  the  Devonian 
beds  of  this  country,  must  have  required  a  thousand  years 
or  more  for  the  deposition  of  the  materials  that  now  oc- 
cupy an  inch  in  depth.  In  those  sections  a  single  foot 
of  the  rock  may  well  represent  a  period  of  ten  thousand 
years.  In  many  of  the  limestones  the  rate  of  accumulation 
could  hardly  have  been  more  speedy.  The  reckoning  has 
to  be  rough,  but  the  impression  which  such  studies  make 
upon  the  mind  of  the  unprejudiced  observer  is  to  the  effect 
that  the  thirty  miles  or  so  of  sedimentary  deposits  could 
not  have  been  formed  in  less  than  a  hundred  million 
years.  In  this  reckoning  it  should  be  noted  that  no  ac- 
count is  taken  of  those  great  intervals  of  unrecorded  time, 
such  as  elapsed  between  the  close  of  the  Laurentian  and 
the  beginning  of  the  Cambrian  periods. 

There  is  a  third  way  in  which  we  may  seek  an  inter- 
pretation of  duration  from  the  rocks.  In  each  successive 
stage  of  the  earth's  history,  in  different  measure  in  the 
various  ages,  mountains  were  formed  which  in.  time,  dur- 
ing their  exposure  to  the  conditions  of  the  land,  were 
worn  down  to  their  roots  and  covered  by  deposits  accu- 
mulated during  the  succeeding  ages.  A  score  or  more 
of  these  successively  constructed  series  of  elevations  may 
readily  be  observed.  Of  old,  it  was  believed  that  moun- 
tain ranges  were  suddenly  formed,  but  there  is,  however, 
ample  evidence  to  prove  that  these  disturbed  portions 
of  the  strata  were  very  gradually  dislocated,  the  rate  of 
the  mountainous  growth  having  been,  in  general,  no 
greater  in  the  past  than  it  is  at  the  present  day,  when,  as 
we  know  full  well,  the  movements  are  going  on  so  slowly 
that  they  escape  observation.  Only  here  and  there,  as 
an  attendant  on  earthquake  shocks  or  other  related  move- 
ments of  the  crust,  do  we  find  any  trace  of  the  upward 
march  which  produces  these  elevations.  Although  not 
a  subject  for  exact  measurements,  these  features  of  moun- 
tain growth  indicate  a  vast  lapse  of  time,  during  which 
the  elevations  were  formed  and  worn  away. 


THE  ROCKS  AND  THEIR  ORDER.  393 

Yet  another  and  very  different  method  by  which 
we  may  obtain  some  gauge  of  the  depths  of  the  past  is 
to  be  found  in  the  steps  which  have  led  organic  life  from 
its  lowest  and  earliest  known  forms  to  the  present  state 
of  advancement.  Taking  the  changes  of  species  which 
have  occurred  since  the  beginning  of  the  last  ice  epoch, 
we  find  that  the  changes  which  have  been  made  in  the 
organic  life  have  been  very  small;  no  naturalist  who  has 
obtained  a  clear  idea  of  the  facts  will  question  the  state- 
ment that  they  are  not  a  thousandth  part  of  the  altera- 
tions which  have  occurred  since  the  Laurentian  time. 
The  writer  is  of  the  opinion  that  they  do  not  represent 
the  ten  thousandth  part  of  those  vast  changes.  These 
changes  are  limited  in  the  main  to  the  disappearance  of 
a  few  forms,  and  to  slight  modifications  in  those  previously 
in  existence  which  have  survived  to  the  present  day.  So 
far  as  we  can  judge,  no  considerable  step  in  the  organic 
series  has  taken  place  in  this  last  great  period  of  the 
earth's  history,  although  it  has  been  a  period  when,  as 
before  noted,  all  the  conditions  have  combined  to  induce 
rapid  modifications  in  both  animals  and  plants.  If,  then, 
we  can  determine  the  duration  of  this  period,  we  may 
obtain  a  gauge  of  some  general  value. 

Although  we  can  not  measure  in  any  accurate  way 
the  duration  of  the  events  which  have  taken  place  since 
the  last  Glacial  period  began  to  wane,  a  study  of  the 
facts  seems  to  show  that  less  than  a  hundred  thousand 
years  can  not  well  be  assumed  for  this  interval.  Some 
of  the  students  who  have  approached  the  subject  are  dis- 
posed to  allow  a  period  of  at  least  twdce  this  length  as 
necessary  for  the  perspective  which  the  train  of  events 
exhibits.  Reckoning  on  the  lowest  estimate,  and  count- 
ing the  organic  changes  w^hich  take  place  during  the  age 
as  amounting  to  the  thousandth  part  of  the  organic 
changes  since  the  Laurentian  age,  we  find  ourselves  in 
face  once  again  of  that  inconceivable  sum  which  was  in- 
dicated by  the  physical  record. 


394:  OUTLINES  OF  THE  EARTH'S  HISTORY. 

Here,  again,  the  critics  assert  that  there  may  have 
been  periods  in  the  history  of  the  earth  when  the  changes 
of  organic  life  occurred  in  a  far  swifter  manner  than  in 
this  last  section  of  the  earth's  history.  This  supposition 
is  inadmissible,  for  it  rests  on  no  kind  of  proof;  it  is, 
moreover,  contraindicated  by  the  evident  fact  that  the 
advance  in  the  organic  series  has  been  more  rapid  in 
recent  time  than  at  any  stage  of  the  past.  In  a  word,  all  the 
facts  with  which  the  geologist  deals  are  decidedly  against 
the  assumption  that  terrestrial  changes  in  the  organic 
or  the  inorganic  world  ever  proceed  in  a  spasmodic  man- 
ner. Here  and  there,  and  from  time  to  time,  local  revolu- 
tions of  a  violent  nature  undoubtedly  occur,  but,  so  far 
as  we  may  judge  from  the  aspect  of  the  present  or  the 
records  of  the  past,  these  accidents  are  strictly  local;  the 
earth  has  gone  forward  in  its  changes  much  as  it  is  now 
advancing.  Its  revolutions  have  been  those  of  order  rather 
than  those  of  accident. 

The  first  duty  of  the  naturalist  is  to  take  Nature  as 
he  finds  it.  He  must  avoid  supposing  any  methods  of 
action  which  are  not  clearly  indicated  in  the  facts  that 
he  observes.  The  history  of  his  own  and  of  all  other 
sciences  clearly  shows  that  danger  is  always  incurred 
where  suppositions  as  to  peculiar  methods  of  action  are 
introduced  into  the  interpretation.  It  required  many 
centuries  of  labour  before  the  students  of  the  earth  came 
to  adopt  the  principle  of  explaining  the  problems  with 
which  they  had  to  deal  by  the  evidence  that  the  earth 
submitted  to  them.  Wherever  they  trusted  to  their  im- 
aginations for  guidance,  they  fell  into  error.  Those  who 
endeavour  to  abbreviate  our  conception  of  geologic  time 
by  supposing  that  in  the  olden  days  the  order  of  events 
was  other  than  that  we  now  behold  are  going  counter  to 
the  best  traditions  of  the  science. 

Although  the  aspect  of  the  record  of  life  since  the 
beginning  of  the  Cambrian  time  indicates  a  period  of  at 
least  a  hundred  million  years,  it  must  not  be  supposed 


THE  ROCKS  AND  THEIR  ORDER.  395 

that  this  is  the  limit  of  the  time  reauired  for  the  develop- 
ment of  the  organic  series.  All  tne  important  types  of 
animals  were  already  in  existence  in  that  ancient  period 
with  the  exception  of  the  vertebrates,  the  remains  of 
which  have  apparently  now  been  traced  down  to  near  the 
Cambrian  level.  In  other  words,  at  the  stage  where  we 
first  find  evidence  of  living  beings  the  series  to  which  they 
belong  had  already  climbed  very  far  above  the  level  of 
lifeless  matter.  Few  naturalists  will  question  the  state- 
ment that  half  the  work  of  organic  advance  had  been 
accomplished  at  the  beginning  of  the  Cambrian  rocks. 
The  writer  is  of  the  opinion  that  the  development  which 
took  place  before  that  age  must  have  required  a  much 
longer  period  than  has  elapsed  from  that  epoch  to  the 
present  day.  We  thus  come  to  the  conclusion  that  the 
measurement  of  duration  afforded  by  organic  life  indi- 
cates a  yet  more  lengthened  claim  of  events,  and  demands 
more  time  than  appears  to  be  required  for  the  formation 
of  the  stratified  rocks. 

The  index  of  duration  afforded  by  the  organic  series  is 
probably  more  trustworthy  than  that  which  is  found  in 
the  sedimentary  strata,  and  this  for  the  reason  that  the 
records  of  those  strata  have  been  subjected  to  numerous 
and  immeasurable  breaks,  while  the  development  of  or- 
ganic life  has  of  necessity  been  perfectly  continuous.  The 
one  record  can  at  any  point  be  broken  without  interrupting 
the  sequences;  the  other  does  not  admit  of  any  breaches 
in  the  continuity. 

The  Moon. 

Set  over  against  the  earth — related  to,  yet  contrasted 
with  it  in  many  ways — the  moon  offers  a  most  profitable 
object  to  the  student  of  geology.  He  should  often  turn 
to  it  for  those  lessons  which  will  be  briefly  noted. 

In  the  beginning  of  their  mutual  history  the  mate- 
rials of  earth  and  moon  doubtless  formed  one  vaporous 
body  which  had  been  parted  from  the  concentrating  mass 


396  OUTLINES  OF  THE  EARTH'S  HISTORY. 

of  the  sun  in  the  maimer  noted  in  the  sketch  of  the  his- 
tory of  the  solar  system.  After  the  earth-moon  body  had 
gathered  into  a  nebulous  sphere,  it  is  most  likely  that  a 
ring  resembling  that  still  existing  about  Saturn  was 
formed  about  the  earth,  which  in  time  consolidated  into 
the  satellite.  Thenceforth  the  two  bodies  were  parted, 
except  for  the  gravitative  attraction  which  impelled  them 
to  revolve  about  their  common  centre  of  gravity,  and  ex- 
cept for  the  light  and  heat  they  might  exchange  with  one 
another. 

The  first  stages  after  the  parting  of  the  spheres  of  earth 
and  moon  appear  to  have  been  essentially  the  same  in  each 
body.  Concentrating  upon  their  centres,  they  became  in 
time  fluid  by  heat;  further  on,  they  entered  the  rigid  state 
— in  a  word,  they  froze — at  least  in  their  outer  parts.  At 
this  point  in  their  existence  their  histories  utterly  diverge; 
or  rather,  we  may  say,  the  development  of  the  earth  con- 
tinued in  a  vast  unfolding,  while  that  of  the  moon  appears 
to  have  been  absolutely  arrested  in  ways  which  w^e  will 
now  describe. 

With  the  naked  eye  we  see  on  the  moon  a  considerable 
variation  in  the  light  of  different  parts  of  its  surface;  we 
discern  that  the  darker  patches  appear  to  be  rudely  cir- 
cular, and  that  they  run  together  on  their  margins.  See- 
ing this  little,  the  ancients  fancied  that  our  satellite  had 
seas  and  lands  like  the  earth.  The  first  telescopes  did  not 
dispel  their  fancies;  even  down  to  the  early  part  of  this 
century  there  were  astronomers  who  believed  the  moon 
to  be  habitable;  indeed,  they  thought  to  find  evidence 
that  it  was  the  dwelling  place  of  intelligent  beings  who 
built  cities,  and  who  tried  to  signal  their  intellectual  kin- 
dred of  this  planet.  When^  however,  strong  glasses  were 
applied  to  the  exploration,  these  pleasing  fancies  were 
rudely  dispelled. 

Seen  with  a  telescope  of  the  better  sort,  the  moon 
reveals  itself  to  be  in  large  part  made  up  of  circular  de- 
pressions, each  surrounded  by  a  ringlike  wall,  with  nearly 


THE  ROCKS  AND  THEIR  ORDER. 


397 


level  but  rough  places  between.  The  largest  of  these 
walled  areas  is  some  four  hundred  miles  in  diameter; 
thence  they  grade  down  to  the  smallest  pits  which  the  glass 
can  disclose,  which  are  probably  not  over  as  many  feet 
across.  The  writer,  from  a  careful  study  of  these  pits,  has 
come  to  the  conclusion  that  the  wider  are  the  older  and 
the  smaller  the  last  formed.  The  rude  elevations  about 
these  pits — some  of  which  rise  to  the  height  of  ten  thou- 
sand feet  or  more — con- 
stitute the  principal  top- 
ographic reliefs  of  the 
lunar  surface.  Besides 
the  pits  above  men- 
tioned, there  are  numer- 
ous fractures  in  the  sur- 
face of  the  plains  and 
ringlike  ridges;  on  the 
most  of  these  the  walls 
have  separated,  forming 
trenches  not  unlike  what 
we  find  in  the  case  of 
some  terrestrial  breaks 
such  as  have  been  noted 
about  volcanoes  and  else- 
where. It  may  be  that  the  so-called  canals  of  Mars  are 
of  the  same  nature. 

The  most  curious  feature  on  the  moon's  surface  are 
the  bands  of  lighter  colour,  which,  radiating  from  certain 
of  the  volcanolike  pits — those  of  lesser  size  and  probably 
of  latest  origin — extend  in  some  cases  for  five  hundred 
miles  or  more  across  the  surface.  These  light  bands  have 
never  been  adequately  explained.  It  seems  most  likely 
that  they  are  stains  along  the  sides  of  cracks,  such  as  are 
sometimes  observed  about  volcanoes. 

The  eminent  peculiarity  of  the  moon  is  that  it  is  desti- 
tute of  any  kind  of  gaseous  or  aqueous  envelope.  That 
there  ig  no  distinct  atmosphere  is  clearly  shown  by  the 


Fig.  23. 


Lunar  mountains  near  the 
Gulf  of  Iris. 


398  OUTLINES  OF  THE  EARTH'S  HISTORY. 

perfectly  sharp  and  sudden  way  in  which  the  hght  of  a 
star  disappears  when  it  goes  behind  the  moon  and  the  clear 
lines  of  the  edge  of  the  satellite  in  a  solar  eclipse.  The 
same  evidence  shows  that  there  is  no  vapour  of  water; 
moreover,  a  careful  search  which  the  writer  has  made  shows 
that  the  surface  has  none  of  those  continuous  down  grades 
which  mark  the  work  of  water  flowing  over  the  land. 
Nearly  all  of  the  surface  consists  of  shallow  or  deep  pits, 
such  as  could  not  have  been  formed  by  water  action.  We 
therefore  have  not  only  to  conclude  that  the  moon  is 
waterless,  but  that  it  has  been  in  this  condition  ever  since 
the  part  that  is  turned  toward  us  was  shaped. 

As  the  moon,  except  for  the  slight  movement  termed 
its  "  libration,"  always  turns  the  same  face  to  us,  so  that 
we  see  in  alj  only  about  four  sevenths  of  its  surface,  it 
has  naturally  been  conjectured  that  the  unseen  side,  which 
is  probably  some  miles  lower  than  that  turned  toward  us, 
might  have  a  different  character  from  that  which  we  be- 
hold. There  are  reasons  why  this  is  improbable.  In  the 
first  place,  we  see  on  the  extreme  border  of  the  moon, 
when  the  libration  turns  one  side  the  farthest  around 
toward  the  earth,  the  edge  of  a  number  of  the  great  walled 
pits  such  as  are  so  plenty  on  the  visible  area;  it  is  fair  to 
assume  that  these  rings  are  completed  in  the  invisible 
realm.  On  this  basis  we  can  partly  map  about  a  third 
of  the  hidden  side.  Furthermore,  there  are  certain  bands 
of  light  which,  though  appearing  on  the  visible  side,  evi- 
dently converge  to  some  points  on  the  other.  It  is  reason- 
able to  suppose  that,  as  all  other  bands  radiate  from  walled 
pits,  these  also  start  from  such  topographic  features.  In 
this  way  certain  likenesses  of  the  hidden  area  to  that  which 
is  visible  is  established,  thus  making  it  probable  that  the 
whole  surface  of  the  satellite  has  the  same  character. 

Clearly  as  the  greater  part  of  the  moon  is  revealed  to 
us — so  clearly,  indeed,  that  it  is  possible  to  map  any  ele- 
vation of  its  surface  that  attains  the  height  of  five  hun- 
drQ^  f?et — th^  interpretation  Qi  its  features  in  the  light 


THE  ROCKS  AND  THEIR  ORDER.  399 

of  geology  is  a  matter  of  very  great  difficulty.  The  main 
points  seem  to  be  tolerably  clear;  they  are  as  follows:  The 
surface  of  the  moon  as  we  see  it  is  that  which  was  formed 
when  that  body,  passing  from  the  state  of  fluidity  from 
heat,  formed  a  solid  crust.  The  pits  which  we  observe  on 
its  surface  are  the  depressions  which  were  formed  as  the 
mass  gradually  ceased  to  boil.  The  later  formed  of  these 
openings  are  the  smaller,  as  would  be  the  case  in  such  a 
slowing  down  of  a  boiling  process. 

As  the  diameter  of  the  moon  is  only  about  one  fourth 
of  that  of  the  earth,  its  bulk  is  only  about  one  sixteenth  of 
that  of  its  planet;  consequently,  it  must  have  cooled  to 
the  point  of  solidification  ages  before  the  larger  sphere 
attained  that  state.  It  is  probable  that  the  same  change- 
less face  that  we  see  looked  down  for  millions  of  years  on 
an  earth  which  was  still  a  seething,  fiery  mass.  In  a  word, 
all  that  vast  history  which  is  traceable  in  the  rocks  be- 
neath our  feet — which  is  in  progress  in  the  seas  and  lands 
and  is  to  endure  for  an  inconceivable  time  to  come — has 
been  denied  our  satellite,  for  the  reason  that  it  had  no 
air  with  which  to  entrap  the  solar  heat  and  no  water  to 
apply  the  solar  energy  to  evolutionary  processes.  The 
heat  which  comes  upon  the  moon  as  large  a  share  for 
each  equal  area  as  it  comes  upon  the  earth  flies  at  once 
away  from  the  airless  surface,  at  most  giving  it  a  tem- 
porary warmth,  but  instituting  no  geological  work  unless 
it  be  a  little  movement  from  the  expansion  and  contraction 
of  the  rocks.  During  the  ages  in  which  the  moon  has 
remained  thus  lifeless  the  earth,  owing  to  its  air  and  water, 
has  applied  a  vast  amount  of  solar  energy  to  geological 
work  in  the  development  and  redevelopment  of  its  geo- 
logical features  and  to  the  processes  of  organic  life.  We 
thus  see  the  fundamental  importance  of  the  volatile  en- 
velopes of  our  sphere,  how  absolutely  they  have  deter- 
mined its  history. 

It  would  be  interesting  to  consider  the  causes  which 
kd  to  the  absence  of  air  and  water  or  the  moon,  but  this 


400  OUTLINES  OF  THE  EARTH'S  HISTORY. 

matter  is  one  of  the  most  debatable  of  all  that  relates  to 
that  sphere;  we  shall  therefore  have  to  content  ourselves 
with  the  above  brief  statements  as  to  the  vast  and  far-act- 
ing effects  which  have  arisen  from  the  non-existence  of 
those  envelopes  on  our  nearest  neighbour  of  the  heavens. 

Methods  in  studying  Geology. 

So  far  as  possible  the  preceding  pages,  by  the  method 
adopted  in  the  presentation  of  facts,  will  serve  to  show 
the  student  the  ways  in  which  he  may  best  undertake 
to  trace  the  order  of  events  exhibited  in  the  phenomena 
of  the  earth.  Following  the  plan  pursued,  we  shall  now 
consider  certain  special  points  which  need  to  be  noted  by 
those  who  would  adopt  the  methods  of  the  geologist. 

At  the  outset  of  his  studies  it  may  be  well  for  the  in- 
quirer to  note  the  fact  that  familiarity  with  the  world 
about  him  leads  the  man  in  all  cases  to  a  certain  neglect 
and  contempt  of  all  the  familiar  presentations  of  Nature. 
We  inevitably  forget  that  those  points  of  light  in  the 
firmament  are  vast  suns,  and  we  overlook  the  fact  that 
the  soil  beneath  our  feet  is  not  mere  dirt,  but  a  marvel- 
lous structure,  more  complicated  in  its  processes  than  the 
chemist's  laboratory,  from  which  the  sustenance  of  our 
own  and  all  other  lives  is  drawn.  We  feel  our  own  bodies 
as  dear  but  commonplace  possessions,  though  we  should 
understand  them  as  inheritances  from  the  inconceivable 
past,  which  have  come  to  us  through  tens  of  thousands 
of  different  species  and  hundreds  of  millions  of  individual 
ancestors.  We  must  overlook  these  things  in  our  common 
life.  If  we  could  take  them  into  account,  each  soul  would 
carry  the  universe  as  an  intellectual  burden. 

It  is,  however,  well  from  time  to  time  to  contemplate 
the  truth,  and  to  force  ourselves  to  see  that  all  this  ap- 
parently simple  and  ordinary  medley  of  the  world  about 
us  is  a  part  of  a  vast  procession  of  events,  coming  forth 
from  the  darkness  of  the  past  and  moving  on  beyond  the 


THE  ROCKS  AND  THEIR  ORDER.  401 

light  of  the  present  day.  Even  in  his  professional  work 
the  naturalist  of  necessity  falls  into  the  commonplace  way 
of  regarding  the  facts  with  which  he  deals.  If  he  be  an 
astronomer,  he  catalogues  the  stars  with  little  more  sense 
of  the  immensities  than  the  man  who  keeps  a  shop  takes 
account  of  his  wares.  Nevertheless,  the  real  profit  of  all 
learning  is  in  the  largeness  of  the  understanding  which  it 
develops  in  man.  The  periods  of  growth  in  knowledge 
are  those  in  which  the  mind,  enriched  by  its  store,  enlarges 
its  conception  while  it  escapes  from  commonplace  ways  of 
thought.  With  this  brief  mention  of  what  is  by  far  the 
most  important  principle  of  guidance  which  the  student 
can  follow,  we  will  turn  to  the  questions  of  method  that 
the  student  need  follow  in  his  ordinary  work. 

With  almost  all  students  a  difficulty  is  encountered 
which  hinders  them  in  acquiring  any  large  views  as  to 
the  world  about  them.  This  is  due  to  the  fact  that  they 
can  not  make  and  retain  in  memory  clear  pictures  of  the 
things  they  see.  They  remember  words  rather  than  things 
' — in  fact,  the  training  in  language,  which  is  so  large  a  part 
of  an  education,  tends  ever  to  diminish  the  element  of  visual 
memory.  The  first  task  of  the  student  who  would  become 
a  naturalist  is  to  take  his  knowledge  from  the  thing,  and 
to  remember  it  by  the  mental  picture  of  the  thing.  In 
all  education  in  Nature,  whether  the  student  is  guided  by 
his  own  understanding  or  that  of  the  teacher,  a  first  and 
very  continuous  aim  should  be  to  enforce  the  habit  of 
recalling  very  distinct  images  of  all  objects  which  it  is 
desired  to  remember.  To  this  end  the  student  should 
practise  himself  by  looking  intently  upon  a  landscape  or 
any  other  object;  then,  turning  away,  he  should  try  to 
recall  what  he  has  beheld.  After  a  moment  the  impression 
by  the  sight  should  be  repeated,  and  the  study  of  the 
memory  renewed.  The  writer  knows  by  his  own  experi- 
ence that  even  in  middle-aged  people,  where  it  is  hard  to 
breed  new  habits,  such  deliberate  training  can  greatly  in- 
crease the  capacity  of  the  memory  for  taking  in  and  repro- 


402  OUTLINES  OF  THE  EARTH'S  HISTORY. 

ducing  images  which  are  deemed  of  importance.  Practice 
of  this  kind  should  form  a  part  of  every  naturalist's  daily 
routine.  After  a  certain  time,  it  need  not  be  consciously 
done.  The  movements  of  thought  and  action  will,  indeed, 
become  as  automatic  as  those  which  the  trained  fencer 
makes  with  his  foil. 

Along  with  the  habit  of  visualizing  memories,  and  of 
storing  them  without  the  use  of  words,  the  student  should 
undertake  to  enlarge  his  powers  of  conceiving  spaces  and 
directions  as  they  exist  in  the  field  about  him.  Among 
savages  and  animals  below  the  grade  of  man,  this  under- 
standing of  spacial  relations  is  very  clear  and  strong.  It 
enables  the  primitive  man  to  find  his  way  through  the 
trackless  forest,  and  the  carrier  pigeon  to  recover  his  mate 
and  dwelling  place  from  the  distance  of  hundreds  of  miles 
away.  In  civilized  men,  however,  the  habit  of  the  home 
and  street  and  the  disuse  of  the  ancient  freedom  has 
dulled,  and  in  some  instances  almost  destroyed,  all  sense 
of  this  shape  of  the  external  world.  The  best  training 
to  recover  this  precious  capacity  will  now  be  set  forth. 

The  student  should  begin  by  drawing  a  map  on  a  true 
scale,  however  roughly  the  w^ork  may  be  done,  of  those 
features  of  the  earth  about  him  with  which  he  is  neces- 
sarily most  familiar.  The  task  may  well  be  begun  with 
his  own  dwelling  or  his  schoolroom.  Thence  it  may  be 
extended  so  as  to  include  the  plan  of  the  neighbouring 
streets  or  fields.  At  first,  only  directions  and  distances 
should  be  platted.  After  a  time  to  these  indications  should 
be  added  on  the  map  lines  indicating  in  a  general  way 
contours  or  the  lines  formed  by  horizontal  planes  inter- 
secting the  area  subject  to  delineation.  After  attaining 
certain  rude  skill  in  such  work,  the  student  may  advan- 
tageously make  excursions  to  districts  which  he  can  see 
only  in  a  hurried  way.  As  he  goes,  he  should  endeavour 
to  note  on  a  sketch  map  the  positions  of  the  hills  and 
streams  and  the  directions  of  the  roads.  A  year  of  holiday 
practice  in  auch  work  will,  if  the  t&sks  occupy  somewhere 


THE  ROCKS  AND  THEIR  ORDER.  403 

about  a  hundred  hours  of  his  time,  serve  greatly  to  ex- 
tend or  reawaken  what  may  be  called  the  topographic 
sense,  and  enable  him  to  place  in  terms  of  space  the  ob- 
servations of  Nature  which  he  may  make. 

In  his  more  detailed  work  the  student  should  select 
some  particular  field  for  his  inquiry.  If  he  be  specially 
interested  in  geologic  phenomena,  he  will  best  begin  by 
noting  two  classes  of  facts — those  exhibited  in  the  rocks 
as  they  actually  appear  in  the  state  of  repose  as  shown  in 
the  outcrops  of  his  neighbourhood,  and  those  shown  in  the 
active  manifestations  of  geological  work,  the  decay  of  the 
rocks  and  the  transportation  of  their  waste,  or,  if  the  con- 
ditions favour,  the  complicated  phenomena  of  the  sea- 
shores. 

As  soon  as  the  student  begins  to  observe,  he  should 
begin  to  make  a  record  of  his  studies.  To  the  novice  in 
any  science  written,  and  particularly  sketched,  notes  are 
of  the  utmost  importance.  These,  whether  in  words  or 
in  drawings,  should  be  made  in  face  of  the  facts;  they 
should,  indeed,  be  set  down  at  the  close  of  an  observation, 
though  not  until  the  observer  feels  that  the  object  he  is 
studying  has  yielded  to  him  all  which  it  can  at  that  time 
give.  It  is  well  to  remark  that  where  a  record  is  made 
at  the  outset  of  a  study  the  student  is  apt  to  feel  that  he 
is  in  some  way  pledged  to  shape  all  he  may  see  to  fit  that 
which  he  has  first  written.  In  his  early  experience  as  a 
teacher,  the  writer  was  accustomed  to  have  students  com- 
pare their  work  of  observation  and  delineation  with  that 
done  by  trained  men  on  the  same  ground.  It  now  seems  to 
him  best  for  the  beginner  at  first  to  avoid  all  such  reference 
of  his  own  work  to  that  of  others.  So  great  is  the  need  of 
developing  independent  motive  that  it  is  better  at  the 
outset  to  make  many  blunders  than  to  secure  accuracy  by 
trust  in  a  leader.  The  skilful  teacher  can  give  fitting 
words  of  caution  which  may  help  a  student  to  find  the 
true  way,  but  any  reference  of  his  undertakings  to  master- 
pieces is  sure  to  breed  a  servile  habit.     Therefore  such 


404  OUTLINES  OP  THE  EARTH'S  HISTORY. 

comparisons  are  fitting  only  after  the  habit  of  free  work 
has  been  well  formed.  The  student  who  can  afford  the 
help  of  a  master,  or,  better,  the  assistance  of  many,  such 
as  some  of  our  universities  offer,  should  by  all  means  avail 
himself  of  this  resource.  More  than  any  other  science, 
geology,  because  of  the  complexity  of  the  considerations 
with  which  it  has  to  deal,  depends  upon  methods  of  labour 
which  are  to  a  great  extent  traditional,  and  which  can 
not,  indeed,  be  well  transmitted  except  in  the  personal 
way.  In  the  distinctly  limited  sciences,  such  as  mathe- 
matics, physics,  or  even  those  which  deal  with  organic 
bodies,  the  methods  of  work  can  be  so  far  set  forth  in 
printed  directions  that  the  student  may  to  a  great  extent 
acquire  sound  ways  of  work  without  the  help  of  a  teacher. 

Although  there  is  a  vast  and  important  literature  con- 
cerning geology,  the  greater  part  of  it  is  of  a  very  special 
nature,  and  will  convey  to  the  beginner  no  substantial  in- 
formation whatever.  It  is  not  until  he  has  become  familiar 
with  the  field  with  which  he  is  enabled  to  deal  in  the  actual 
way  that  he  can  transfer  experience  thus  acquired  to  other 
grounds.  Therefore  beyond  the  pleasing  views  which  he 
may  obtain  by  reading  certain  general  works  on  the  sci- 
ence, the  student  should  at  the  outset  of  his  inquiry  limit 
his  work  as  far  as  possible  to  his  field  of  practice,  using 
a  good  text-book,  such  as  Dana's  Manual  of  Geology,  as  a 
source  of  suggestions  as  to  the  problems  which  his  field 
may  afford. 

The  main  aim  of  the  student  in  this,  as  in  other 
branches  of  inquiry,  is  to  gain  practice  in  following  out 
the  natural  series  of  actions.  To  the  primitive  man  the 
phenomenal  world  presents  itself  as  a  mere  phantasmago- 
ria, a  vast  show  in  which  the  things  seen  are  only  related  to 
each  other  by  the  fact  that  they  come  at  once  into  view. 
The  end  of  science  is  to  divine  the  order  of  this  host,  and 
the  ways  in  which  it  is  marshalled  in  its  onward  movement 
and  the  ends  to  which  its  march  appears  to  be  directed. 
So  far  as  the  student  observes  well,  and  thus  gains  a  clear 


THE  ROCKS  AND  THEIR  ORDER.  405 

notion  of  separated  facts,  he  is  in  a  fair  way  to  gather  the 
data  of  knowledge  which  may  be  useful;  but  the  real 
value  of  these  discernments  is  not  gained  until  the  obser- 
vations go  together,  so  as  to  make  something  with  a  per- 
spective. Until  the  store  of  separate  facts  is  thus  arranged, 
it  is  merely  crude  material  for  thought;  it  is  not  in  the 
true  meaning  science,  any  more  than  a  store  of  stone  and 
mortar  is  architecture.  When  the  student  has  developed 
an  appetite  for  the  appreciation  of  order  and  sources  of 
energy  in  phenomena,  he  has  passed  his  novitiate,  and 
becomes  one  of  that  happy  body  of  men  who  not  only  see 
what  is  perceived  by  the  mass  of  their  fellows,  but  are 
enabled  to  look  through  those  chains  of  action  which, 
when  comprehended,  serve  to  rationalize  and  ennoble  all 
that  the  senses  of  man,  aided  by  the  instruments  which  he 
has  devised,  tell  us  concerning  the  visible  world. 


INDEX 


JEtna,  Mount,  381. 

Agriculture,  American,  346  ;  in  Eng- 
land, winning  swamp  lands  for, 
335 ;  recent  developments  of,  345. 

Alaska,  changes  on  the  coast  of,  96. 

Ants  taking  food  underground,  319 ; 
work  of  the,  on  the  soil,  318. 

Apsides,  revolution  of  the,  61,  62. 

Arabians,  chemical  experiments  of 
the,  13. 

Arches,  natural,  in  cavern  districts, 
258. 

Artesian  wells,  258,  259. 

Arts,  advance  of  Italian  fine,  19. 

Asteroids,  53 ;  motions  of,  about  their 
centres  and  about  the  sun,  53. 

Astronomers,  the  solar  system  and 
the  early,  79. 

Astronomy,  31-80;  growth  of,  since 
the  time  of  Galileo,  33,  34 ;  the  first 
science,  10. 

Atmosphere,  97-206  ;  along  the  trop- 
ical belt,  102;  as  a  medium  of 
communication  between  different 
regions,  99  ;  deprived  of  water,  con- 
taining little  heat,  105;  beginning 
of  the  science  of  the,  117  ;  counter- 
trade movements  of  the,  105 ;  en- 
velope of  the  earth,  98 ;  expansion 
of,  in  a  hollow  wall  during  the  pas- 
sage of  a  storm,  114 ;  heat-carrying 
power  of  the,  105 ;  heights  to  which 
it  extends,  99 ;  in  water,  99 ;  move- 
ments no  direct  infiuence  on  the 
27 


surface  of  the  earth,  122 ;  move- 
ments of  the,  qualified  by  the  con- 
dition which  it  encounters,  118;  of 
mountains,  98;  of  the  seashore,  98; 
of  the  earth,  98 ;  of  the  sun,  73 ; 
snow  as  an  evidence  of,  65 ;  supply- 
ing needs  of  underground  creatures, 
331 ;  uprushes  of,  101, 102 ;  upward 
strain  of  the,  next  the  earth,  107 ; 
weight  and  motion  of  the,  120, 
121. 

Atmospheric  circulation  of  the  soil, 
330,  331 ;  envelopes,  97. 

Aurora  boreal  is,  168. 

Avalanches,  210-213 ;  dreaded,  in  the 
Alpine  regions,  212;  great,  in  the 
Swiss  Oberland,  211,  212;  rocky, 
175-177. 

Axis,  imaginary  changes  in  the 
earth's,  59  ;  of  the  earth's  rotation, 
58 ;  polar,  inclined  position  of,  58 ; 
polar,  nodding  movement  of  the 
axes,  54 ;  rotations  of  the  planetary 
spheres  on  their  axes,  56. 

Barometer,  causes  of  changes  in  the, 

117, 118. 
Basalts,  309. 
Beaches,  93,  142,  144;   boulder,  142, 

143  ;  pebbly,  142 ;  sand,  144. 
Beetles,  work  of,  on  the  soil,  318,  319, 
Belief  of  the  early  astronomers  about 

the  solar  system,  79. 

Bergschrund,  the,  214. 

407 


4u8 


OUTLINES  OF  THE  EARTH'S  HISTORY. 


Birds  and  mammals  contributing  to 
the  fertility  of  the  soil,  319. 

"  Blanketing,"  269. 

Bogs,  climbing,  331-334;  lake,  331- 
333 ;  peat,  334,  335 ;  quaking,  334. 

Botany,  rapid  advance  in,  14,  15. 

Boulders,  217,  220. 

Breakers,  135, 137, 139.  , 

Bridges,  natural,  257,  258. 

Canals  of  Mars,  67. 

Canon,  newly  formed  river  cutting  a, 
195. 

Cataracts,  193. 

Caves,  253-258,  261 ;  architecture  of, 
255-258 ;  hot- water,  261 ;  mammoth 
cave,  258;  stalactites  and  stalag- 
mites on  the  roof  and  floor  of,  257. 

Chasms,  140, 141. 

Chemistry,  6,  12, 14;  advance  of,  12; 
modern,  evolving  from  the  studies 
of  alchemists,  13, 14. 

Chromosphere,  73. 

Civilization  of  the  Icelanders,  384. 

Clifls,  sea-beaten,  132, 141, 142. 

Climate,  changes  of,  due  to  modifica- 
tions of  the  ocean  streams,  153 ; 
effect  of  the  ocean  on  the,  147 ;  of 
the  Gulf  Stream,  149,  150. 

Clouds,  1^9 ;  formation  of,  162, 163 ; 
shape  of,  163;  water  of,  usually 
frozen,  207 ;  cloud-making,  laws  of, 
161, 162. 

Coast,  changes  on  the  Scandinavian, 
96 ;  line,  effect  of  tide  on  the,  145 ; 
of  Greenland,  226;  of  New  Jersey 
sinking,  95  ;  marine,  changes  in, 
95. 

Cold  in  Siberia,  243. 

Comets,  47,  50  ;  collisions  of,  50 ;  kin- 
ship of  meteorites  and,  48 ;  omens 
of  calamity  to  the  ancients,  50 ;  the 
great,  of  1811,  49,  50. 

Cones.    See  under  Voloakoes. 

Conflict  between  religion  and  science, 
20,    22;    between    the    Protestant 


countries  and  the  followers  of  sci- 
ence, 20. 

Continental  shelves,  125. 

Continents  and  oceans,  83 ;  changes 
in  position  of,  91 ;  cyclones  of  the, 
111 ;  forms  of,  90  ;  proofs  that  they 
have  endured  for  many  years,  92 ; 
shape  of,  84,  96. 

Coral  reefs,  153,  353. 

Corona,  realm  of  the,  73. 

Craters.    See  under  Volcanoes. 

Crevasse,  a  barrier  to  the  explorer,  218. 

Crevice  water,  250. 

Curds,  214. 

Currents,  coral  reefs  in  Florida  affect- 
ing the  velocity  of,  153 ;  equatorial, 
150;  of  the  Gulf  Stream,  147-149; 
hot  and  cold,  of  the  sea,  102  ;  ocean, 
145 ;  oceanic  action  of  trade  winds 
on,  145 ;  effect  on  migration  of, 
157 ;  icebergs  indicating,  243 ;  sur- 
face, history  of,  172 ;  uprushing, 
near  the  equator,  106. 

Cyclones,  111 ;  cause  of.  111 ;  of 
North  America,  111;  secondary 
storms  of,  112. 

Deltas,  173, 187. 

Deposits,  vein,  260,  261. 

Deserts,  interior,  158. 

Dew,  159,  160;  a  concomitant  of 
cloudless  skies,  160,  and  vegetation, 
160 ;  formation  of,  159-161. 

Diablerets,  174. 

Diagram  of  a  vein,  260;  showing 
development  of  swamp,  335 ;  how  a 
portion  of  the  earth's  surface  may 
be  sunk  by  faulting,  374;  growth 
of  mangroves,  340  ;  the  effect  of  the 
position  of  the  fulcrum  point  in 
the  movement  of  the  land  masses, 
94. 

Diameter  of  our  sphere  at  the  equa- 
tor, 62 ;  of  the  earth,  82. 

Dikes,  192,  293;  305-310;  abounding 
in    volcanic    cones,    305;    cutting 


INDEX. 


409 


through  coal,  306  •,  driven  upward, 
307  ;  formation  of,  305,  310 ;  mate- 
rial of,  307, 308 ;  representing  move- 
ments of  softened  rock,  309 ;  their 
relation  to  volcanic  cones,  307  ;  va- 
riations of  the  materials  of,  307, 
308;  waterfalls  produced  by,  192; 
zone  of,  306. 

Dismal  Swamp,  95,  333. 

Distances,  general  idea  of,  27 ;  good 
way  to  study,  27,  28 ;  training  sol- 
diers to  measure,  28. 

Doldrums,  104,  109;  doldrum  of  the 
equator,  109 ;  of  the  hurricane,  109. 

Drainage,  imperfect,  of  a  country  af- 
fected by  glaciers,  242. 

Dunes,  123,  124, 325,  326,  387  ;  mould- 
ed, 387. 

Duration  of  geological  time,  389. 

Dust  accumulations  from  wind,  in 
China,  122. 

Earth,  a  flattened  sphere,  82;  air  en- 
velope of  the,  98 ;  amount  of  heat 
falling  from  the  sun  on  the,  41 ;  an- 
tiquity of  the,  391 ;  atmosphere  of 
the,  98;  attracting  power  of  the, 
127 ;  axis  of  the  rotation  of  the,  58 ; 
composition  of  the  atmosphere  of 
the,  98;  crust  of  the,  aftected  by 
weight,  93;  deviation  of  the  path 
of  the,  varied,  61 ;  diameter  of  the, 
82  ;  of  the,  affected  by  loss  of  heat, 
131 ;  difference  in  altitude  of  the 
surface  of  the,  83;  discovery  that 
it  was  globular,  31,  32;  effect  of 
imaginary  changes  in  the  relations 
of  sun  and,  59 ;  effect  of  the  interior 
heat  of  the,  309,  310 ;  effect  of  the 
sun  on  the,  60,  61 ;  formerly  in  a 
fluid  state,  82;  imaginary  view  of 
tJie,  from  the  moon,  81 ;  important 
feature  of  the  surface  of  the,  83; 
jarring  caused  by  faults,  367 ;  sur- 
face of  the,  determined  by  heat  and 
light  from  the  sun,  57;  most  im- 


portant feature  of  the  surface  of 
the,  83 ;  motion  of  the,  affecting 
the  direction  of  trade  winds,  103 ; 
movements,  366  ;  natural  architec- 
ture of  the,  377 ;  no  part  of  the,  ex- 
empt from  movement,  384 ;  parting 
of  the  moon  and,  396 ;  path  of  the, 
around  the  sun,  55,  56,  59,  60 ;  re- 
volving from  east  to  west,  103 ; 
shrinking  of  the,  from  daily  escape 
of  heat,  89 ;  soil-covering  of  the,  343 ; 
study  of  the,  81-96  ;  swaying,  385 ; 
tensions,  problem  of,  371 ;  tremors, 
caused  by  chemical  changes  in  the 
rocks,  385 ;  tropical  belt  of  the,  74 ; 
viewed  from  the  surface  of  the 
moon,  311,  312;  water  store  of  the, 
125. 
Earthquakes,  277,  278,  280,  356,  358, 
370-384,  388-390  ;  accidents  of,  358 ; 
action  of,  356 ;  agents  of  degrada- 
tion, 383,  384 ;  basis  of,  367  ;  certain 
limitations  to,  380, 381 ;  Charleston, 
of  1883,  374,  375 ;  countries,  archi- 
tecture in,  381 ;  echoes,  369,  370 ; 
damages  of,  377,  390 ;  ettect  of,  on 
the  soil,  375;  the  surface  of  the 
earth,  371 ;  formed  by  riving  of  fis- 
sures, 382  ;  great,  occurring  wliere 
rocks  have  been  disturbed  by 
mountain-building,  381,  382;  Iler- 
culaneum  and  Pompeii  destroyed 
by  an,  277,  280 ;  Italian,  in  1783, 
371,  372;  important,  not  connected 
with  volcanic  explosions,  381 ;  Ja- 
maica, in  1692,  372,  376  ;  Lisbon,  in 
1755,  368,  369,  373,  374,  381 ;  maxi- 
mum swing  of,  369  ;  measuring  the 
liability  to,  386,  387 ;  mechanism  of, 
370,  371 ;  method  of  the  study  of, 
followed  by  Mr.  Charles  Mallet,  382, 
383;  Mississippi,  in  1811,  373,  374, 
380,  381 ;  movement  of  the  earth 
during,  377;  originating  from  a 
fault  plane,  367,  369,  370 ;  originat- 
ing from  the  seas,  358,  375 ;  oscilla- 


410 


OUTLINES  OF  THE  EARTH'S  HISTORY. 


tion  of,  376 ;  poised  rocks  indicating 
a  long  exemption  from  strong,  388 ; 
Eiobamba,  in  17974  375 ;  shocks  of, 
and  their  effect  upon  people,  383; 
the  direct  calamities  of  Nature,  386  ; 
waves  of,  389. 

Earthworms,  317-319;  taking  food 
underground,  319. 

Eclipses,  record  of  ancient,  130. 

Electrical  action  in  the  formation  of 
rain  and  snow,  164. 

Elevations  of  seas  and  lands,  83. 

Energy  indestructible,  23. 

Envelope,  lower,  of  the  sun,  74. 

Equator,  diameter  of  our  sphere  at 
the,  62 ;  doldrum  of  the,  109 ;  up- 
draught  under  the,  102 ;  uprushing 
current  near  the,  106. 

Equinoxes,  precession  of  the,  61,  62. 

Eskers^  221. 

Expansion  of  air  contained  in  a  hol- 
low wall  during  the  passage  of  the 
storm,  114. 

Experiment,  illustrating  consolida- 
tion of  disseminated  materials  of 
the  sun  and  planets,  40. 

Falls.    See  Waterfalls. 

Fault  planes,  382. 

Feldspar,  324. 

Floods,  180,  197;  rarity  of,  in  New 

England,  121 ;  river,  frequent  east 

of  Rocky  Mountains,  198. 
Fohns,  121. 
Forests,  salicifled,  124. 
Fossilization,  354-356. 
Fulcrum  point,  95. 

Galactic  plane,  45. 

Galonggoon,  eruption  of,  294. 

Geological  work  of  water,  168-206. 

Glacial  action  in  the  valleys  of  Swit- 
zerland, 224 ;  periods,  63,  243,  246 ; 
in  the  northern  hemisphere,  246; 
waste,  324. 

Glaciation,  effect  of,  in  North  Amer- 


ica, 241 ;  in  Central  America,  234 ; 
South  America,  234. 

Glaciers,  207-249 ;  action  of  ice  in 
forming,  230-232;  Alaskan,  216; 
continental,  225,  239,  240 ;  discharge 
of,  220 ;  exploring,  220 ;  extensive, 
in  Greenland  and  Scandinavia,  244 ; 
former,  of  North  America,  232,  234 ; 
map  of,  and  moraines  near  Mont 
Blanc,  217 ;  motions  of,  213  ;  retreat 
of  the,  228,  230,  235  ;  secrets  of  the 
under  ice  of,  221 ;  speed  of  a,  224 ; 
study  of,  in  the  Swiss  valleys,  222; 
testimony  of  the  rocks  regarding, 
228 ;  when  covered  with  winter 
snows,  216  ;  valley,  216. 

Gombridge,  1830,  74. 

Gravitation,  law  of,  4. 

Greeks'  idea  of  the  heavens,  31 ;  not 
mechanically  inventive,  22. 

Gulf  Stream,  current  of  the,  147. 

Ileat,  amount  of,  daily  escaping  from 
the  earth,  89 ;  amount  of,  falling 
from  the  sun  on  the  earth,  41 ;  be- 
lief of  the  ancients  regarding,  42 ; 
dominating  effect  on  air  currents  of 
tropical,  104 ;  energy  with  which  it 
leaves  the  sun,  41 ;  internal,  of  the 
earth,  88,  89  ;  of  the  earth's  interi- 
or, 309,  310 ;  sun,  effect  on  the  at- 
mosphere of  the,  100 ;  Prof.  New- 
comb's  belief  regarding  the,  of  the 
sun,  52 ;  radiation  of  the  earth's, 
causing  winds,  101 ;  solar,  41 ; 
tropical,  and  air  currents,  104. 

Hills,  sand,  123. 

Horizontal  pendulum,  384. 

Horse  latitudes,  104. 

"Horses,"  261. 

Hurricanes,  107, 110,  317  ;  commence- 
ment of,  107  ;  doldrum  of,  109  ;  felt 
near  the  sea,  110;  in  the  tropics, 
110. 

Hypothesis,  nebular,  34, 35, 39, 52, 56 ; 
working,  4,  5. 


INDEX. 


411 


Ice  action,  effect  of  intense,  222,  223  ; 
in  forming  glaciers,  230,  232 ;  re- 
cent studies  in  Greenland  of,  239  5 
depth  of,  in  Greenland,  227 ;  effect 
of,  on  river  channels,  196;  effect 
of,  on  stream  beds,  196  ;  expanding 
when  freezing,  237  ;  epoch,  92,  93, 
246 ;  floating,  242 ;  made  soils  rare- 
ly fertile,  241 ;  mass,  greatest,  in 
Greenland,  226,  227;  moulded  by 
pressure,  215;  streams,  continental, 
225,  226  ;  of  the  mountains,  225 ;  of 
the  Himalayan  Mountains,  234. 

Icebergs,  242,  243  ;  indicating  oceanic 
currents,  243. 

Iceland,  volcanic  eruptions  in,  297, 
298. 

Instruments,  first,  astronomical,  10, 11. 

Inventions,  mechanical,  aiding  sci- 
ence, 22. 

Islands,  84,  272 ;  continental,  84 :  in 
the  deeper  seas  made  up  of  volcanic 
ejections,  272 ;  volcanic,  272. 

Jack-o'-lantern,  167. 

Jupiter,  gaseous  wraps  of,  97 ;  path  of 
the  earth  affected  by,  59,  60;  the 
largest  planet  of  the  sun,  69. 

Kames,  325. 

Kant,  Iramanuel,  and  nebular  hy- 
pothesis, 34. 

Kaolin,  324. 

Klondike  district,  cold  in,  243,  244. 

Krakatoa,  eruption  of,  298-300 ;  eftect 
of,  on  the  sea,  299 ;  efi'ect  of,  on  the 
sun,  300. 

Lacolites,  306. 

Lacustrine  beds,  351. 

Lagoons,  salt  deposits  found  in,  200. 

Lake  basins,  formation  of,  200,  201 ; 

bogs,  331,  333,  334;  deposits,  350, 

351. 
Lakes,  199-206 ;  effect  of,  on  the  river 

system,     205;    fresh-water,     145; 


formed  from  caverns,  202;  great, 
changing  their  outlets,  205 ;  of  ex- 
tinct volcanoes,  203;  temporary 
features  of  the  land,  203 ;  volcanic, 
203. 

Lands,  great,  relatively  unchange- 
able, 96 ;  table,  91  ;  movements  re- 
sulting in  change  of  coast  line,  351, 
352 ;  shape  of  the  seas  and,  83,  84 ; 
accounting  for  the  changes  in  the 
attitude  of  the,  95 ;  and  water,  di- 
visions of,  84 ;  dry,  surface  of,  85  ; 
general  statement  as  to  the  division 
of  the,  83,  84 ;  surface,  shape  of  the, 
85 ;  triangular  forms  of  great,  90. 

Latitudes,  horse,  troublesome  to 
mariners,  104. 

Laplace  and  nebular  hypothesis,  34. 

Lava,  266-268,  270,  271,  292,  293,  295, 
296,  303,  304;  flow  of,  invading  a 
forest,  268  ;  from  Vesuvius,  293  ;  of 
1669,  295,  296;  temperature  of,  295, 
296;  incipient,  304;  outbreaks  of, 
292,  303 ;  stream  caves,  292,  293. 

Law,  natural,  Aristotle  and,  3 ;  of 
gravitation,  4 ;  of  the  conservation 
of  energy,  23. 

Leaves,  radiation  of,  160. 

Length  of  days  affected  by  tidal  ac- 
tion, 131. 

Level  surfaces,  91. 

Life,  organic,  evolution  of,  15, 16. 

Light,  belief  of  the  ancients  regard- 
ing, 42. 

Lightning,  24,  164-168;  noise  from, 
166;  proceeding  from  the  earth  to 
the  clouds,  165  ;  protection  of  build- 
ings from,  165  ;  stroke,  wearing-out 
effect  of,  165. 

Limestones,  353,  357,  358,  360,  364; 
formation  of,  357,  360. 

Lisbon,  earthquake  of,  1755,  368,  369. 

Lowell,  Mr.  Percival,  observations  on 
Venus,  64. 

Lunar  mountains  near  the  Gulf  oi 
Iris,  397. 


412 


OUTLINES  OF  THE  EARTH'S  HISTORY. 


Mackerel  sky,  35. 

Mallet,  Mr.  Charles,  and  the  study  of 
earthquakes,  382,  383. 

Man  as  an  inventor  of  tools,  10. 

Mangroves,  340;  diagram  showing 
mode  of  groM^th,  340  ;  marshes  of, 
339. 

Map  of  glaciers  and  moraines  near 
Mont  Blanc,  217;  of  Ipswich 
marshes,  338. 

Mapping  with  contour  lines,  27. 

Maps,  desirable,  for  the  study  of  ce- 
lestial geography,  77 ;  geographic 
sketch,  26,  27. 

Marching  sands  jeopardizing  agricul- 
ture, 123. 

Marine  animals,  sustenance  of,  361- 
363;  deposits,  325-327,  349,  356; 
marshes,  336-340 ;  waves  caused  by 
earthquakes,  387. 

Mars,  65-67,  84,97 ;  belief  that  it  has 
an  atmosphere,  65;  canals  of,  67; 
gaseous  wraps  of,  97 ;  more  efficient 
telescopes  required  for  the  study  of, 
67 ;  nearer  to  the  earth  than  other 
planets,  65. 

Marshes,  mangrove,  339 ;  map  of 
Ipswich,  338 ;  marine,  336-340 ;  de- 
posits found  in,  336;  of  North 
America,  337 ;  on  the  coast  of  New 
England,  339;  phenomena  of,  167, 
168 ;  tidal,  good  earth  for  tillage, 
337 ;  tidal,  of  North  America,  340. 

Mercury,  55,  63,  78;  nearest  to  the 
sun,  63 ;  time  in  which  it  completes 
the  circle  of  its  year,  55. 

Meteorites,  47,  48 ;  kinship  of  comets 
and,  48. 

Meteors,  47 ;  falling,  47 ;  composition 
of,  48 ;  flashing,  39,  40,  47  ;  speed  of, 
47  ;  inflamed  by  friction  with  air,  99. 

Methods  in  studying  geology,  400. 

Milky  Way,  45;  voyage  along  the 
path  of  the,  44,  45. 

Mineral  crusts,  328,  329 ;  deposits,  308. 

Moon,  38,  395-400 ;  absence  of  air  and 


water  on  the,  399 ;  attended  by  sat' 
ellites,  57 ;  attraction  which  it  ex- 
ercises on  the  earth,  62;  curious 
feature  of  the,  397  ;  destitute  of  gas- 
eous or  aqueous  envelope,  397 ;  di- 
ameter of  the,  399  ;  imaginary  view 
of  the  earth  from  the,  81 ;  "  libra- 
tion"  of  the,  398;  made  up  of  cir- 
cular depressions,  396,  397;  move- 
ments of  the,  78  ;  no  atmosphere  in 
the,  97;  parting  of  the  earth  and, 
396 ;  position  of  the,  in  relation  to 
the  earth,  62 ;  tidal  action  and  the, 
131;  tides  of  the,  126,  127;  why 
does  the  sun  not  act  in  the  same 
manner  as  the,  78. 

Moraines,  216,218,229,  230;  map  of 
glaciers  and,  near  Mont  Blanc,  217  ; 
movements  of  the,  216-218;  termi- 
nal, 228. 

Moulin^  219. 

Mounrt  ^tna,  288-310 ;  lava  yielding, 
290,  293,  294 ;  lava  stream  caves  of, 
292,  293;  more  powerful  than  Ve- 
suvius, 297:  peculiarities  of,  291, 
292 ;  size  of,  289-291 ;  turning  of  the 
torrents  of,  295, 

Mountain-building,  90-93,  304  ;  fold- 
ing, 86,  87,  90,  365;  attributed  to 
cooling  of  the  earth,  88;  growth, 
392;  Swiss  falls,  174;  torrents,  en- 
ergy of,  177. 

Mountains,  85,  86,  89,  90-93  ;  174-178; 
form  and  structure  of,  86 ;  partly 
caused  by  escape  of  heat  from  the 
earth,  89  ;  sections  of,  87. 

Mount  Nuova,  formation  of,  284. 

Mount  Vesuvius,  263-285,  288, 289,  293, 
S02,  381 ;  description  of  the  eruption 
of,  in  A.  D.  79, 277-280 ;  diagrammatic 
sections  through,  showing  changes 
in  the  form  of  the' cone,  283;  erup- 
tion of,  in  1056,  281 ;  in  1882-'83,  264, 
266 ;  eruption  of,  in  1872,  282 ;  erup- 
tions of,  increased  since  1636,  282; 
flow  of  lava  from,  285;  likely   to 


INDEX. 


413 


enter  on  a  period  of  inaction,  282, 
283;   outbreak  of,  in  1882-'83,  264, 


Naples,  prosperity  of  the  city,  289. 
Nebular  hypothesis,  34,  35,  39,  52. 
Neptune,  70. 
iVeW,the,  214;  no  ice-cutting  in  the 

region  of  the,  224. 
Newcomb's  (Prof.)  belief  regarding 

the  heat  of  the  sun,  52. 
Niagara  Falls,  191,  192,  204;  cutting 

back  of,  204. 
North  America,  changes  in  the  form 

of,  91,  92 ;  triangular  form  of,  90. 

Ocean,  average  depth  of  the,  89 ;  cli- 
matal  effect  of  the,  147;  currents, 
145;  effect  of,  on  migration,  156; 
effect  of,  on  organic  life,  154 ;  floor, 
85,  93 ;  hot  and  cold  currents  of 
the,  102 ;  sinking  of  the,  93,  94 ;  the 
laboratory  of  sedimentary  deposits, 
351 ;  depth  of  the,  89, 126. 

Oceanic  circulation,  effect  of,  on  the 
temperature,  152. 

Oceans  and  continents,  83. 

Orbit,  alterations  of  the,  and  the  sea- 
sons, 60,  61 ;  changing  of  the,  59- 
63  ;  shape  of  the,  61-63. 

Organic  life,  315,  317,  321,  352,  353, 
363 ;  action  of,  on  the  soil,  317, 321 ; 
advantages  of  the  shore  belt  to,  363 ; 
development  of  in  the  sea,  352, 353 ; 
effect  of  ocean  currents  on,  154; 
processes  of,  in  the  soil,  315 ;  decay 
of,  in  the  earth,  321. 

Orion,  46. 

Oscillations  of  the  shores  of  the  Bay 
of  Naples,  287. 

Oxbow  of  a  river,  182, 183. 

Oxbows  and  cut-off,  182. 

Pebbles,  action  of  seaweeds  on,  143 ; 

action  of  the  waves  on,  142, 144. 
Photosphere,  74, 


Plains,  86;  alluvial,  91,  179,  182, 184- 
186,325;  history  of,  91 ;  sand,  325. 

Planets,  38 ;  attended  by  satellites, 
57;  comparative  sizes  of  the,  68; 
experiments  illustrating  consolida- 
tion of  disseminated  materials  of 
the  sun  and,  40 ;  gaseous  wraps  of, 
97  ;  important  observations  by  the 
ancients  of  fixed  stars  and  planets, 
43 ;  movements  of,  57-61 ;  outer,  78 ; 
table  of  relative  masses  of  sun  and, 
77. 

Plant  life  in  the  Sargassum  basins, 
156. 

Plants  and  animals,  protection  of,  by 
mechanical  contrivances,  364  ;  and 
trees,  work  of  the  roots  of,  on  the 
soil,  316,  317;  water-loving,  181; 
forming  climbing  bogs,  332. 

Polar  axes,  nodding  movement  of,  54. 

Polar  snow  cap,  66. 

Polyps,  155,  353. 

Pools,  circular,  203. 

Prairies,  340,  342. 

Radiation  of  heat,  159. 

Kain,  152, 156,  164,  168,  170,  328,  330; 
circuit  of  the,  156-168 ;  drops,  force 
of,  169,  170;  spheroidal  form  of, 
170 ;  electrical  action  in  the  forma- 
tion of  snow  and,  164 ;  work  of  the, 
171. 

Kealm,  unseen  solar,  75. 

Reeds,  332. 

Religion,  conflict  between  science 
and,  20,  22;  struggle  between  pa- 
ganism and,  21. 

Rivers  and  debris^  183;  changes  in 
the  course  of,  in  alluvial  plain, 
182;  deposition  of,  accelerated  by 
tree-planting,  181 ;  great,  always 
clear,  205 ;  inundation  of  the  Mis- 
sissippi, eating  away  land,  182; 
muds,  222;  newly  formed,  cutting 
a  canon,  195 ;  of  snow-ice,  211 ;  ori- 
gin of  a  normal,  173 ;  oxbow  of  a, 


4U 


OUTLINES  OF  THE  EARTH'S   HISTORY. 


182,  183;  sinking  of,  199;  swinging 
movement  of,  179-181 ;  river-val- 
leys, 193, 194;  diversity  in  the  form 
of  188-191. 

Rocks,  145;  accidents  from  falling, 
174;  cut  away  by  sandstones,  188; 
divided  by  crevices,  252 ;  duration 
of  events  recorded  in,  389,  390 , 
ejection  of,  material,  811;  falling 
of,  174-176  ;  formation  of,  262,  263 ; 
from  the  present  day  to  the  strata 
of  the  Laurentian,  390 ;  migration 
of,  291 ;  poised,  indicating  a  long 
exemption  fronj  strong  earthquakes, 
388;  rents  in,  252,  253;  stratifica- 
tion of,  349,  350,  352, 365,  390 ;  testi- 
mony of  the,  in  regard  to  glaciers, 
228  ;  under  volcanoes,  303  ;  variable 
elasticity  of,  366  ;  vibration  of,  367, 
368;  rock-waste,  march  of  the,  343  ; 
water,  250,  267. 

Kotation  of  the  earth  affected  by  tides, 
130;  of  the  planetary  spheres  on 
their  axes,  56. 

Salicifled  forests,  124. 

Salt  deposits  formed  in  lagoons,  200 ; 
found  in  lakes,  199-200. 

Sand  bars,  183 ;  endurance  of,  against 
the  waves,  145  ;  hills,  travelling  of, 
123 ;  marching,  123 ;  silicious  stones 
cutting  away  rocks,  188. 

Satellites,  53,  54;  motions  of,  about 
their  centres  and  about  the  sun, 
53,  54. 

Saturn,  38,  53,  57,  396 ;  cloud  bands 
of,  70 ;  gaseous  wraps  of,  97 ;  path 
of  the  earth  affected  by,  59,  60. 

Savages,  primitive,  students  of  Na- 
ture, 1. 

Scandinavia,  changes  on  the  coasts 
of,  96. 

Science,  advance  of,  due  to  mechan- 
ical inventions,  22 ;  astronomy  be- 
ginning with,  10;  chemical,  char- 
^9^§n^U9^  of,  Hj  conflict  between 


religion  and,  20,  22;  conflict  be- 
tween the  Koman  faith  and,  20; 
mechanical  inventions  as  aids  to, 
22,  23  ;  modern  and  ancient,  4 ;  nat- 
ural, 5,  6 ;  of  botany  in  Aristotle's 
time,  14 ;  of  physiology,  15  ;  of  zool- 
ogy in  Aristotle's  time,  14;  resting 
practically  on  sight,  10. 

Scientific  development,  historic  out- 
lines of,  17 ;  tools  used  in  measuring 
and  weighing,  as  an  aid  to  vision, 
12. 

Sea,  battering  action  of  the,  140 ;  coast 
ever  changing,  385,  386  ;  effect  of 
volcanic  eruptions  on  the,  299 ;  floor 
deposits  of  the,  affected  by  volca- 
noes, 360, 361 ;  in  receipt  of  organic 
and  mineral  matter,  359  ;  hot  and 
cold  currents  of  the,  102;  littoral 
zone  of  the,  351, 352 ;  puss,  142 ;  rich 
in  organic  life,  352,  353  ;  solvent  ac- 
tion of  the,  361 ;  strata,  formation  of, 
354 ;  water,  minerals  in,  185 ;  weeds, 
155, 156. 

Seas,  dead,  originally  living  lakes, 
200;  water  of,  buoyant,  199;  even- 
tually the  seat  of  salt  deposits,  199- 
201 ;  general  statement  as  to  division 
of,  83,  84 ;  shape  of  the,  83,  84. 

Seashore,  air  of  the,  98. 

Seasons,  changing  the  character  of 
the,  61,  62. 

Sense  of  hearing,  9, 10  ;  of  sight,  10 ; 
of  smell,  9,  10 ;  of  taste,  9,  10 ;  of 
touch,  9, 10. 

Seracs,  214. 

Shocks,  earthquake.  See  under 
Earthquakes. 

Shore  lines,  variation  of,  83,  84. 

Shores,  cliff,  138-142. 

Sink  holes,  202 ;  in  limestone  dis- 
tricts, 253,  254. 

Skaptar,  eruption  of,  297,  298;  lava 
from  the  eruption  of,  298. 

Sky,  mackerel,  35. 

Sqqw,  207-225,  244  j  ft§  m  §vi4§n«« 


INDEX. 


415 


of  atmosphere,  65 ;  blankets,  early 
flowers  beginning  to  blossom  un- 
der, 208;  covering,  difference  be- 
tween an  annual  and  perennial, 
210 ;  effect  of,  on  plants,  208 ;  elec- 
trical action  in  the  formation  of 
rain  and,  164;  flakes,  formation  of, 
164;  red,  210;  slides,  210;  slides, 
phenomena  of,  210,  211. 

Soil,  alluvial,  321,  322;  atmospheric 
circulation  of,  330,  331 ;  conditions 
leading  to  formation  of,  313,  331 ; 
continuous  motion  of  the,  314 ;  cov- 
ering of  the  earth,  343 ;  decay  of 
the,  314,  315;  degradation  of  the, 
344-348  ;  means  for  correcting,  346- 
348;  destruction  in  grain  fields 
greater  than  the  accumulation,  344 ; 
developing  on  lava  and  ashes  an 
interesting  study,  343;  develop- 
ment of,  in  desert  regions,  340 ; 
effect  of  animals  and  plants  on  the, 
317-320 ;  effect  of  earthquakes  on 
the,  375:  fertility  of  the,  distin- 
guished from  the  coating,  344,  345  ; 
fertility  of,  aflected  by  rain,  327 ; 
formation  of,  314-321 ;  glacial,  char- 
acteristics of,  324 ;  glaciated,  323, 
324;  irrigation  of  the,  328-330;  lo- 
cal variation  of,  327;  mineral,  321; 
of  arid  regions  fertile  when  sub- 
jected to  irrigation,  341 ;  of  dust  or 
blown  sand,  321 ;  of  immediate 
derivation,  321,  322 ;  phenomena, 
31 3 ;  processes  of  organic  life  in  the, 
315 ;  variation  in,  321-331 ;  vegeta- 
tion protecting  the,  316,  317  ;  wash- 
ing away  of  the,  346,  347 ;  w^in- 
ning,  from  the  sea,  337 ;  work  of 
ants  on  the,  318 ;  tiller,  duty  of  the, 
348. 

Solar  bodies,  general  conditions  of 
the,  63-71 ;  forces,  action  of,  on  the 
earth,  349;  system,  52,  56;  inde- 
pendent from  the  fixed  stars  sys- 
tem, 4?  ;  Qngleal  vapour  of,  52, 53 ; 


singular  features  of  our,  68;  tide, 
127. 

Spheres,  difference  in  magnitude  of, 
51 ;  motions  of  the,  50,  51 ;  plane- 
tary, rotation  of,  on  their  axes,  56. 

Spots,  sun,  72. 

Spouting  horn,  141. 

Springs,  formation  of  small,  252. 

Stalactitization,  256. 

Stalagmites  and  stalactites  on  the 
roof  and  floor  of  a  cavern,  257. 

Stars  as  dark  bodies  in  the  heavens, 
47 ;  discovery  of  Fraunhofer  and 
others  on,  23,  33  ;  double,  39 ;  and 
tidal  action,  131 ;  earliest  study  of, 
10;  fixed,  important  observations 
by  the  ancients  of  planets  and,  43 ; 
not  isolated  suns,  38,  39 ;  variation 
in  the  light  of,  46 ;  limit  of,  seen  by 
the  naked  eye,  11 ;  revolution  of 
one  star  about  another,  46,  47 ; 
shooting,  47  ;  speed  of  certain,  51 ; 
study  of,  31  80 ;  sudden  flashing 
forth  of,  due  to  catastrophe,  46; 
voyage  through  the,  44,  45 ;  star, 
wandering,  74. 

Stellar  realm,  31-80. 

Storms,  circular.  111 ;  desert,  121, 
122;  expansion  of  air  contained  in 
a  hollow  wall  during  the  passage 
of,  114 ;  great  principle  of,  105, 106 ; 
in  the  Sahara,  121 ;  lightning,  more 
frequent  in  summer,  167  ;  paths  of, 
115;  secondary,  of  cyclones,  112; 
spinning,  115;  thunder,  165-167; 
whirling,  106,  124;  whirling  pe- 
culiarity of,  108,  109. 

Strabo,  writings  of,  18. 

Sun,  atmosphere  of  the,  73;  consti- 
tution of  the,  72 ;  distance  of  the 
earth  from  the,  29 ;  effect  from 
clianges  in  the,  and  earth,  59 ;  en- 
velope of  the,  73,  74,  97;  experi- 
ments illustrating  consolidation  of 
disseminated  materials    of  planets 

ftnd,  40  5  fiu^Hj^,  a^rk  mi  wld,  42; 


416 


OUTLINES  OF  THE  EARTH'S  HISTORY. 


formation  of  the  eight  planets  of 
the,  53 ;  heat  leaving  the,  41 ;  heat 
of  the,  76 ;  imaginary  journey  from 
the,  into  space,  44 ;  mass  of  the, 
76,  77;  path  of  the  earth  around 
the,  55 ;  physical  condition  of  the, 
71 ;  Prof.  Newcomb's  belief  regard- 
ing the  heat  of  the,  52 ;  spots,  75 ; 
abundant  at  certain  intervals,  72 ; 
difficulty  in  revealing  cause  of,  75 ; 
structure  of  the,  a  problem  before 
the  use  of  the  telescope,  72 ;  table 
of  relative  masses  of,  and  planets, 
77;  three  stages  in  the  history  ot 
the,  71 ;  tides,  126 ;  why  does  it 
not  act  in  the  same  maimer  as  the 
moon  ?  78. 

Surfaces,  level,  90. 

Surf  belt,  swayings  of  the,  137. 

Swamps,  diagram  showing  remains 
of,  335 ;  Dismal  Swamp,  95,  333 ; 
drainage  of,  334,  335 ;  fresh-water, 
334,  335 ;  phenomena  of,  167,  168. 

Table-lands,  91. 

Table  of  relative  masses  of  sun  and 
planets,  77. 

Telescopes,  11, 12,  45;  first  results  of, 
72 ;  power  of,  11 ;  revelations  of,  45. 

Temperature,  effects  of,  produced  by 
vibration,  42 ;  in  the  doldrum  belt, 
118 ;  of  North  America,  118 ;  of  the 
Atlantic  Ocean,  118. 

Tempests,  rate  of,  99,  100. 

Thunder,  166;  more  pronounced  in 
the  mountains,  166. 

Thunderstorms,  165,  166;  distribu- 
tion of,  166,  167. 

Tidal  action,  recent  studies  of,  131, 
132;  marslies  of  North  America, 
340. 

Tides,  carving  channels,  129  ;  effect- 
ing the  earth's  rotation,  130;  effect 
of,  on  marine  life,  130;  height  of, 
128, 129 ;  moon  and  sun,  126, 127  ; 
normal  run  of  the,  127 ;  production 


of,  131;  of  the  trade  winds,  150; 
solar,  127 ;  travelling  of,  127, 128. 

Tillage  introducing  air  into  the  pores 
of  the  soil,  331. 

Tornadoes,  112,  113,  317;  develop- 
ment of,  113 ;  effect  of,  on  build- 
ings, 113 ;  fiercest  in  North  Amer- 
ica, 113 ;  length  of,  115 ;  resem- 
blance of,  to  hurricanes,  115;  up- 
sucking  action  of,  114, 115. 

Torrents,  177-179,  204. 

Trade  winds.    See  under  Winds. 

Training  in  language,  diminishing 
visual  memory,  401 ;  soldiers  to 
measure  distances,  28;  to  measure 
intervals  of  time,  28;  for  a  natu- 
ralist, 25-29. 

Tunnels,  natural,  257. 

Uranus,  70. 

Valley  of  Val  del  Bove  formed  from 
disturbances  of  Mount  Jiltna,  294. 

Valleys,  diversity  in  the  form  of  river, 
188-191 ;  river,  193. 

Vapour,  156,  157,  159,  163;  gravita- 
tive  attraction  of,  34,  35;  nebular 
theory  of,  52,  53 ;  original,  of  the 
solar  system,  52,  53. 

Vegetation,  and  dew,  160 ;  in  a  meas- 
ure, independent  of  rain,  160;  pro- 
tecting the  soil,  316,  317. 

Vein,  diagram  of  a,  260. 

Venus,  64,  78  ;  recent  observations  of, 
by  Mr.  Percival  Lowell,  64. 

Vesuvian  system,  study  of  the,  285. 

Vesuvius.    See  Mount  Vesuvius. 

Visualizing  memories,  402,  403. 

Volcanic  action,  268-276. 

Volcanic  eruption  of  a.  d.  79,  288; 
important  facts  concerning,  276- 
279;  islands,  272;  lava  a  primary 
feature  in,  266;  observations  of, 
made  from  a  balloon,  301 ;  peaks 
along  the  floor  of  the  sea,  272,  273; 
possibility  of  throwing  matter  be- 


INDEX. 


417 


yond  control  ot  gravittitive  energy^ 
300. 
Volcanoes,  125,  203,  2t)3;  abounding 
on  the  sea  floor,  302 ;  accidents 
from  eruptions  of,  288;  along  the 
Pacific  coast,  271 ;  ash  sliowers  of, 
maintaining  fertility  of  the  soil, 
289;  distribution  of,  271;  eruption 
of,  286-294,  368;  explosions  from, 
coming  from  a  supposed  liquid  in- 
terior of  the  earth,  275 ;  exporting 
earth  material,  310;  little  water, 
375 ;  Italian,  considered  collective- 
ly, 296,  297  ;  l^eapolitan  eruptions  of 
and  the  history  of  civilization,  288 ; 
subsidence  of  the  earth  after  erup- 
tion of,  287,  291 ;  origin  of,  263-274-, 
phenomena  of,  263-267  ;  submarine, 
301 ;  travelling  of  ejections  from, 
287,  288. 

Waters,  crevice,  250 ;  of  the  earth,  250, 
251;  cutting  action  of,  117,  192; 
drift,  from  the  poles,  151 ;  journey 
of,  from  the  Arctic  Circle  to  the 
tropics,  151, 152;  dynamic  value  of, 
171 ;  expansion  of,  in  rocks,  270 ; 
geological  work  of,  168-206 ;  in  air, 
99;  of  the  clouds  usually  frozen, 
207;  pure,  no  power  for  cutting 
rocks,  204;  rock,  250,  263;  sea, 
minerals  in,  185 ;  store  of  the  earth, 
125;  system  of,  125,  156;  tropical, 
151;  velocity  of  the,  under  the 
equator,  150 ;  wearing  away  rocks, 
178,  179;  underground,  carrying 
mineral  matter  to  the  sea,  193; 
chemical  changes  of,  leading  to 
changes  in  rock  material,  262,  263; 
effect  of  carbonic-acid  gas  on,  251 ; 
operations  of  the,  126 ;  wearing 
away  rocks,  178, 179  ;  work  of,  250. 

Waterfalls,  189-193;  cause  of,  191; 
the  Yosemite,192 ;  Niagara,  191, 192; 
numerous  in  the  torrent  district  of 


rivers,  192;  produced  by  dikes, 
192;  valuable  to  manufactures,  192, 
193. 

Waterspouts,  115,  116;  atmospheric 
cause  of,  116  ;  tiring  at,  116  ;  life  of 
a,  116;  picturesqueness  of,  116;  the 
water  of  fresh,  117. 

Waves,  128,  129,  132,  145 ;  action  of 
friction  on,  135, 136  ;  break  of  the, 
136 ;  endurance  of  sand  against  the, 
145 ;  force  of,  133, 136, 139 :  marine, 
caused  by  eartliquakes,  387  ;  of 
earthquakes,  389  ;  peculiar  features 
in  the  action  of,  137 ;  size  of,  137, 
138;  stroke  of  the,  144;  surf,  135; 
tidal  height  of,  132 ;  undulations  of, 
132;  wind,  132;  wind  influence  of, 
on  the  sea,  134, 135  ;  wind-made,  128. 

Ways  and  means  of  studying  Na- 
ture, 9. 

Weeds  of  the  sea,  155. 

Well,  artesian,  258,  259. 

Whirling  of  fluids  and  gas,  36,  37. 

Whirlwinds  in  Sahara,  121. 

Will-o'-the-wisp,  167. 

Winds,  101,  110,  122,  317 ;  eff'ect  of 
sand,  122  ;  hurricane,  110  ;  illustra- 
tion of  how  they  are  produced,  101 ; 
in  Martha's  Vineyard,  120;  of  the 
forests,  work  of  the,  317 ;  of  torna- 
does, effect  of,  113 ;  on  the  island  of 
Jamaica,  119,  120 ;  regimen  of  the, 
119 ;  variable  falling  away  in  the 
nighttune,  100;  trade,  102-105; 
145,  146,  150 ;  action  of,  on  ocean 
currents,  145 ;  affected  by  motion  of 
tlie  earth,  103  ;  belt,  motion  of  the 
ocean  in,  146 ;  flow  and  counter- 
flow  of  the,  150  ;  tide  of  the,  150  ; 
uniform  condition  of  the,  102; 
waves,  work  of,  132, 134,  135. 

Witchcraft,  belief  of,  in  the  early 
ages,  21. 

Zoology,  rapid  advance  in,  14, 15. 


(8) 


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inquiry,  and  while  the  findings  of  that  court  were  for  the  most  part  in  accord- 
ance with  the  results  of  his  own  historical  investigations,  he  has  modified 
certain  portions  of  his  narrative.  Whatever  opinions  may  be  held  regarding 
any  phases  of  our  recent  naval  history,  the  fact  remains  that  the  industry, 
care,  and  thoroughness  which  were  unanimously  praised  by  newspaper  re- 
viewers and  experts  in  the  case  of  the  first  two  volumes  have  been  sedulously 
applied  to  the  preparation  of  this  new  edition  of  the  third  volume. 

A  History  of  American  Privateers. 

Uniform  with  "  A  History  of  the  United  States  Navy." 
One  volume.  Illustrated.  8vo.  Cloth,  $3.00  net ;  post- 
age, 24  cents  additional. 

After  several  years  of  research  the  distinguished  historian  of  American  sea 
power  presents  the  first  comprehensive  account  of  one  of  the  most  picturesque 
and  absorbing  phases  of  our  maritime  warfare.  The  importance  of  the  theme 
is  indicated  by  the  fact  that  the  value  of  prizes  and  cargoes  taken  by  privateers 
in  the  Revolution  was  three  times  that  of  the  prizes  and  cargoes  taken  by 
naval  vessels,  while  in  the  War  of  1812  we  had  517  privateers  and  only  23 
vessels  in  our  navy.  Mr.  Maclay's  romantic  tale  is  accompanied  by  reproduc- 
tions of  contemporary  pictures,  portraits,  and  documents,  and  also  by  illus- 
trations by  Mr.  George  Gibbs. 


D.    APPLETON    AND    COMPANY,    NEW    YORK 


TWENTIETH  CENTURY  TEXT  BOOKS. 

A  History  of  the  American  Nation. 

3y  Andrew  C.  McLaughlin,  Professor  of 
American  History  in  the  University  of  Michi- 
gan. With  many  Maps  and  Illustrations.  1 2mo. 
Cloth,  $1.40. 

<*  One  of  the  most  attractive  and  complete  one-volume  his- 
tories of  America  that  has  yet  appeared.*' — Boston  Beacon, 

**  Complete  enough  to  find  a  place  in  the  library  as  well  as  in 
the  school.** — Deliver  Republican, 

**This  excellent  work,  although  ir.tended  for  school  use,  is 
equally  good  for  general  use  at  home.** — Boston  Transcript, 

**It  should  find  a  place  in  all  historic  libraries.*' — Toledc 
Blade, 

"Clearness  is  not  sacrificed  to  brevity,  and  an  adequate 
knowledge  of  political  causes  and  effects  may  be  gained  from  thi^ 
Concise  history.** — New  York  Christian  Advocate, 

"A  remarkably  good  beginning  for  the  new  Twentieth  Cen- 
tury Series  of  text-books.  .  .  .  The  illustrative  feature,  and 
especially  the  maps,  have  received  the  most  careful  attention, 
and  a  minute  examination  shows  them  to  be  accurate,  truthful, 
and  ''^wsxxz.iwz.'*^ -^Philadelphia  Press, 

"The  work  is  up  to  date,  and  in  accord  with  the  best  modern 
methods.  It  lays  a  foundation  upon  which  a  superstructure  of 
historical  study  of  any  extent  may  be  safely  built.** — Pittsburg 
Times. 

«*A  book  of  rare  excellence  and  practical  usefulness.'* — Saii 
Lake  Tribune, 

"The  volume  is  eminently  worthy  of  a  place  in  a  series  des- 
tined for  the  j-eaders  of  the  coming  century.  It  is  highly 
creditable  to  the  author.** — Chicago  Evening  Post, 

D.    APPLETON     AND     COMPANY,     NEW     YORK. 


COLUMBUS   AND   WASHINGTON- 

The  Story  of  Columbus. 

By  Elizabeth  Eggleston  Seelye.  Edited  by  Dr. 
Edward  Eggleston.  With  loo  Illustrations  by  Allegra 
Eggleston.     Delights  of    History  Series.      i2mo.     Cloth, 

$1.75. 

"  This  is  no  ordinary  work.     It  is  preeminently  a  work  of  the  present  time  and 
of  the  future  as  well." — Boston  Traveler, 

"  Mrs.  Seelye's  book  is  pleasing  in  its  general  effect,  and  reveals  the  results  of 
painstaking  and  conscientious  study." — New  York  Tribune. 


"Ave 
nor  magnii 


!ry  just  account  is  given  of  Columbus,  his  failings  being  neither  concealed 
ined,  but  his  real  greatness  being  made  plain." — New  York  Examiner. 

"  The  illustrations  are  particularly  well  chosen  and  neatly  executed,  and  they 
add  to  the  general  excellence  of  the  volume."— A'^«;  York  Times. 

"  A  brief,  popular,  interesting,  and  yet  critical  volume,  just  such  as  we  should 
wish  to  place  in  the  hands  of  a  young  reader.  The  authors  of  this  volume  have 
done  their  best  to  keep  it  on  a  high  plane  of  accuracy  and  conscientious  work  with- 
out losing  sight  of  their  readers." — New  York  Indipendent. 

The  Story  of  Washington. 

By  Elizabeth  Eggleston  Seelye.  Edited  by  Dr. 
Edward  Eggleston.  With  over  loo  Illustrations  by 
Allegra  Eggleston.  Delights  of  History  Series.  i2mo. 
Cloth,  $1.75. 

"One  of  the  best  accounts  of  the  incidents  of  Washington's  life  for  young 
people." — New  York  Observer. 

*'  The  Washington  described  is  not  that  of  the  demigod  or  hero  of  the  first  half 
of  this  century,  but  the  man  Washington,  with  his  defects  as  well  as  his  virtues,  his 
unattractive  traits  as  well  as  his  pleasing  ones.  .  .  .  There  is  greater  freedom 
from  errors  than  in  more  pretentious  lives." — Chicago  Tribune. 

"  The  illustrations  are  numerous,  and  actually  illustrate,  including  portraits  and 
views,  with  an  occasional  map  and  minor  pictures  suggestive  of  the  habits  and  cus- 
toms of  the  period.  It  is  altogether  an  attractive  and  useful  book,  and  one  that 
should  find  many  readers  among  American  boys  and  girls." — Philadelphia  Times. 

"  Will  be  read  with  interest  by  young  and  old.  It  is  told  with  good  taste  and 
accuracy,  and  if  the  first  President  loses  some  of  his  mythical  goodness  in  this  story, 
the  real  greatness  of  his  natural  character  stands  out  distinctly,  and  his  example  will 
be  all  the  more  helpful  to  the  boys  and  girls  of  this  generation." — New  York 
Churchman. 

D.    APPLETON    AND    COMPANY,     NEW    YORK. 


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